Because: when likes are turned off I feel like I need to comment instead, which makes things even noisier in forum threads.
You do not really need to comment, if you 'like' a post, you can drop a merit, if you do not have any smerits, you can suggest the post to other members or merit sources
here. Any low substance reply, like; "thank you" or "good job" would likely be deleted.
But that's why a like button would be helpful. It's common courtesy to thank someone for help and a like button would at least curb people just littering threads with pleasantries. Sometimes a thanks is all some people require but it's a bit of a stretch for people to go suggesting people merit a post because they don't have any. A simple like button would do the job there.
SMF forum software doesn't have support for like buttons, especially the older version that's used on this forum (not sure about the newer SMF version, I believe those have reactions).
Well it didn't have support for merit either but theymos implemented it

.