"Rest up while you still can!" "Enjoy every minute of your pregnancy, because you're going to miss it!" "Just relax! Everything will be OK!" Stuff like this just falls out of people's mouths the moment they see a pregnant woman. It's like a reflex or something; people apparently can't help themselves, even though these are all bits of "advice" every pregnant woman dreads hearing.
Now, most of the folks saying this stuff probably mean well, particularly if they're parents themselves and it’s been a while since they had their babies. Over time, people's happiness and love for our kids tends to outshine their memories of how hard pregnancy and childbirth can be, and nostalgia for the adorable, tiny past seriously distorts folks' perceptions of pregnancy. Even now, barely two years after my most recent pregnancy, I find myself getting so excited and even mildly jealous of pregnant moms, and I hated being pregnant. I liked giving birth better than pregnancy, yet even I have to actively remind myself of all the crappy parts of pregnancy so I can think of something useful to say to my pregnant friends, and stop myself from turning into a blathering belly-rubber when faced with an expecting mom.
Then, of course, there are the people who have never birthed a baby, but have heard all manner of random pregnancy myths and weird pregnancy sayings from other people, and who feel compelled to say this kind of stuff because they've observed that these are just "things you say" to pregnant ladies. Apparently, a lot of people forget that you can talk to pregnant women about other things that have nothing to do with pregnancy or babies, because pregnant women are still people who have entire lives' worth of interests that don't revolve around gestation. (You don't have to strain to make awkward small talk about baby stuff if that's not your thing, folks! There's a whole world of other conversation topics out there that won't make you sound like a total weirdo, or irritate pregnant strangers, acquaintances, and friends.)
On the plus side, even if you don't currently have the energy to do much else, at least your facial muscles are getting a workout, mama. So many side-eyes and bemused grins and grimaces are bound to pay off, right?