Rick C-137 has made it clear that he disrespects time travel, and a Rick and Morty fan theory claims it's for a somewhat dark reason.
Rick Sanchez is not a fan of time travel, and the genius Rick and Morty's theory explains why. Throughout five seasons, audiences learned a list of things Rick didn't like, including but not limited to bureaucracy, family therapy, Jerry, and time travel. As the righteous narcissist, Rick has an explanation for why he doesn't like both of these things.
For a show where sci-fi seems to know no bounds, Rick and Morty doesn't deal much with time travel. This has to do with the fact that Justin Roiland and Dan Harmon, creators of Rick and Morty, believe that time travel can be a little tricky when adding Pandora's Box to the story. However, the show can't escape the concept of time travel, mainly because of the number of times that intersects with alternate realities and multiverse themes so common with Rick and Morty. This is why it should be possible to find an in-universe explanation of why Rick, as a character, doesn't like time travel.
In Season 4, Episode 8, "The Vat of Acid," Rick gives an explanation for why he disrespects time travel: "If Ant-Man and the Wasp can do it, I don't care." However, according to fan theory (via Reddit), there is a much deeper explanation for Rick's grudge against time travel. Rick C-137 has been looking for the Rick who killed Diane and Beth for a long time - and since he couldn't find him, there's a chance that's the case because he did, not the alternate universe Rick. According to the theory, not having his time machine and not traveling through time too much is how Rick can be sure he's not guilty.
Rick can create a time machine at any time - but he avoids it
Given all of Rick's inventions, putting the time machine together is an easy task. In fact, in "Battlestar Ricklactica", Rick and Morty have to make sure that the snake community (re)discovers time travel eventually, and the duo quickly build their time machine to solve the problem. Rick also has the infamous Time Travel Objects chest in his garage, which means he has at least a few tools to create his time machine whenever he wants.
However, it does not appear that Rick's Portal Gun has been equipped with specific time travel technology. While the full extent of the Portal Gun's powers remains a mystery, there has never been a moment in Rick and Morty where the Portal Gun has been explicitly used for time travel. While one could argue that the Rick and Morty pilot showed Rick opening a portal to the future to find a quick fix for Morty's broken legs, Rick specifically uses the term "future dimension" in the episode, rather than just "future", which seems to have been coined specifically to emphasize the fact that Rick is not going to the future of their universe.
There are many times that Rick had the opportunity to solve a time travel issue during the show's run, but he specifically chose not to. In "ABC's of Beth," for example, he goes through a portal to order pizza in some universe where their favorite place is still open and returns pizza on hand immediately. However, Rick's detailed commentary made it clear to the audience that he wasn't time travel, and that he simply stole pizza from the counter. In "The Vat of Acid," the device that allowed Morty to "reset" events wasn't messing over time, but rather moving Morty through different realities.
Rick and Morty already broke the "time travel rule" (but not with Rick)
Despite the clear, unwritten rule against time travel, Rick and Morty have dealt with this concept more than once. In "Something Ricked This Way Comes," one of the many artifacts sold from the Needful Things store was a pair of gloves that transported the user back to 1936. At the premiere of Rick and Morty 2, "A Rickle in Time," Rick At the end of season one, Morty and Summer have to deal with the consequences of freezing time. Of course, their neglect results in time cracking itself and splitting into multiple timescales. The group's impact overtime was so great that Time Cop Shleemypants had to step in and put Rick into a time prison, confirming that time travel was something well-established and common in the world of Rick and Morty. Time travel will once again be at the center of a story in the aforementioned "Rattlestar Ricklactica," the episode about the snake community inventing time travel. However, on all of these occasions, Rick has never intentionally chosen to travel through time - let alone carry a time machine with him.
Why Rick Doesn't Really Like Time Travel: Theory Explained
Rick can be quite stubborn when he doesn't like something, but simply disrespecting time travel doesn't seem like enough reason to justify avoiding using this resource on all the occasions he would have helped. Rick seems to have a very clear personal reason for his opposition to time travel, and the aforementioned fan theory explains it perfectly. Rick might be horrified by the idea that it was a future version of himself that killed his family, and the only way he can ensure that this doesn't happen is to avoid using time travel as much as possible.
If owning a time machine and going into the past and the future becomes as commonplace for Rick as the multidimensional travel in Rick and Morty, there is no guarantee for him that he is not responsible for Diane and Beth's deaths. It's worth noting that Rick didn't use his scanner - which he uses to test if Rick is the killer - the Rick he's looking for - on himself during the show. This means that the possibility that C-137 himself killed Diane and Beth, although unlikely, has not been completely ruled out - adding to the theory that Rick wants to avoid time travel at all costs to continue making sure it's not him who did that. Of course, just scanning itself will bring up the answer faster, but that's probably one of the few pieces of information on Rick and Morty that Rick's C-137 wasn't ready to get.