I feel like each Steam sale is worse than the last. Is it that we've already bought everything we care about, the discounts aren't as deep or have publishers and devs realized they're just selling themselves short most of the time? Or all of the above?
There are three major factors:
1. You've already got most of the games you really want. The industry can't produce a steady stream of
MUST HAVE games as fast as sales come up, so eventually everything that's left is just stuff that sort of sounds interesting and you don't get excited about that.
2. Bundles run year round and drive down the expected price of games, as well as filling your library with games you are
sort of interested in. Most publishers aren't going to put "real" games on sale for $0.50 - $1 yet I can constantly buy games for that price all year round... making even $7.50 seem expensive and something like $40 seem insane. The added backlog also makes them look worse, 50% off might have been a good price before but now you've got 500 games in your backlog.... might as well just wait and see if it gets a better sale (or gets bundled).
3. Flash sales have gone away. Previously steam would have developers pick two prices - regular sale and flash sale. They didn't want to set the "regular" price too high, because they didn't know if they were going to be in the flash sale or not, while they also wanted a good flash sale price so they would be more likely to get picked. So they'd do something like say 50%/75%. Without flash sales, most devs just split the difference and do something like 66%, which makes sales look much worse than a front page full of 75% off even though the average sale price for all games is probably the same or even lower.