The simple answer is you can't, it's a feature.
It's actually that steam only automatically downloads your most recent played games immediately. Other games are set for future update depending on how long ago you last played them. This started happening about 2+ years ago that I could recall. It's designed that way to relieve bandwidth on game updates.
EG: Path of Exile has a major season update each few months. I have it installed but no longer play it (like I guess a lot of people do with a bunch of games). It delivers the update to people who need it first. Mine is normally set to update days, even weeks later.
I do however wish they would add a "Download all now" button. I'm one of those people that clicks things to get rid of the notifications because they don't look neat/clean and I have hundreds of games installed at once, and clicking each update becomes annoying lol.