Okay, fine. I first want to dispute the premise that the old(er) legends did a lot of time trialing. Rather, they were racing in premier races with rabbits in an era where they had lots of competition capable of running eye-popping times. You HAD to run fast to win races in the 90s and early 00s.
As for why Mo doesn't prioritize getting his PRs down?
1) All races are slower today. If you look at the all time list for 10k, there are 15 people ahead of Farah. But only seven won their races; the other 8 were either second or third! So people were typically getting pushed AND pulled to faster times in a way that doesn't happen anymore. (Debate the reasons for this all you want: less EPO, more money on the roads, fewer track 10ks.) The situation isn't all that different in the 5k. Other than that crazy Paris race in 2012 (imagine being 6th place with a 12:49!), the all time list is dominated by fairly old times. And even if Farah decided he wanted to set up a special PR attempt, a good rabbit is no substitute for a couple of 12:50 guys who are really gunning for you.
2) Nobody in the world today is going to threaten any records from 3-10k, so at best Mo would just be trying to move up the all time list. That's not really that appealing.
3) In our era when records are not going to fall, the world in general starts to care less about time. Is anyone going to really care if Farah wins a DL 5k in 12:48? Of course not.
In any event, I'm not inclined to count his PRs against him, at least with regard to runners that he actually beats who may have gone faster at some point. As for guys from another era, it's tough to say. I'm fairly certain that distance running was fueled by rampant doping in the 90s and early 00s. I'd also be surprised if Farah were NOT doping, but he would never be able to get away with the same kind of stuff. Remember there literally was no EPO test in the 1990s, and even after the test was developed, it wasn't very good at first.