They were an option for some of them. Not most. And you know what else wasn't massively successful until Valve started to take it seriously with Steam? Most everything else about PC gaming. Once upon a time, people would have laughed at the idea of somebody playing games over the internet and having strangers throw money at them for the privilege of watching.
It's only telling a fraction of the story when people say "donations haven't worked." They haven't really been tried yet. Not really. I went back through some of the mods I had installed, very few of them had donation links at all, and only 1 had it prominently displayed. Largely because there was no real expectation for financial compensation until now. You can't say something hasn't worked before it has really been tried in earnest.
If Steams suddenly pops up with say a donation button here:
for all mods, across all games, then we can start to see if it works or not. But, that mod right there, does that guy accept donations? I dunno. I could go to his personal page and find out. But it's not information that is easily accessible.