It's because once a company is a market leader, it isn't anymore about "What feature is useful", but about "What will increase our revenue by x%?". Will adding a multiple index feature add anything to revenue? I doubt it.
It's more about PMs thinking "Which feature will make it look like I'm doing something useful?"
Photoshop has been quietly getting worse for years now. Certain annoyances haven't been touched for decades, but new features - like the one that forces you to save a copy for certain formats even when you don't want to - are so unwelcome and pointless they've have to add a switch in the preferences which allows you to turn them off.
When a product matures and has monopoly lock-in it's essentially frozen. The core workflow will stop improving and updates will usually add unwelcome friction-inducing cruft around the edges.
