As others have noted, smartphones - the flagships, at least, have basically been as fast as needed for many years now.
So what's the point?
Well there's a very good point. This competition (whether with a company's own previous generations, or a competitor) keeps pushing those numbers up until we get to new inflection points. Points at which new things become possible that we just haven't been able to do before. It's not about incrementally making your web pages load faster!
Take the Apple Silicon transition, for example. That happened off the back of the years of investment in the A series chips for iOS devices. If the A series had plateaued when it was "good enough" would the M series have been ready when it was, if ever?
We also have AR/ VR stuff coming (which is more about GPU, but raw CPU will likely help there, too). Perhaps other devices we don't know about yet.
Each year we get new iPhones (and iPads) that have a bundle of new features, or new designs. Improved CPU is only one of those, and hasn't really been the selling point for many years. But if that number didn't go up we'd complain! (look at what happened when the base iPhone 14 got last year's chip - and that's not even the flagship!)
That might not be entirely rational, but for now it helps fuel the other advancement towards making the next big thing possible - so let's not rock the boat
And that's before we talk about efficiency and trickle down to smaller chips - like the Watch chips - which could still do with some advancement. The pressure to move to smaller process sizes impacts both of those in positive ways - but it all comes from the flagship end, where the money is made.