I'm not going to bore you with overly dramatic and unnecessary reasoning.
You spawn in with random, shitty gear. You're up against potentially armed-to-the-teeth PMCs. You cannot lose off a scav run, making the gameplay much more relaxing. You can only gain.
The shit gear dictates your engagements. Winning engagements with shitty gear makes you feel so much better than with a fully decked M4 with practically negative recoil.
In short, scav runs are fun. They let you chad without fear of losing a ton of gear. They force you to think about your engagements because your gear can't compete with the other players. And yeah, some engagements will be impossible to win, but who cares? You don't lose anything and have the potential to hit massive profit runs (Reserve especially upon raider kills).
Bear in mind that this is an opinion. This is neither correct nor incorrect. Just my opinion. If you agree, let me know in the comments.
As always, have fun in your games and I wish much profit upon you.