I guess because from a user point of view it doesn't make much sense: by the time the battery is dead, the phone is so outdated it serves no use anymore. And this comes as someone using old smartphones (still on their first battery).
Being able to easily swap in a brand new battery at the half way point in a phone's lifetime and get back to full day usage so typical overnight charging instead of needing a second mid-day charge cycle, is a huge win.
That you think a 3 or 4 year old phone is outdated when the makers support it for up to twice that long is ludicrous, and even then it's silly to have to toss it because the maker won't provide further security updates. I'd use a phone 10 years, about what I use my laptops for, if the battery was easily replaceable. Instead, they land in a drawer every 3-4 years when the battery isn't good enough and a the cost of manufacturer replacement in dollars and downtime is so exorbitant that I just buy a new device.
More like 3-4 for android vendors, with the exception of google, and it’s a new thing for them.
https://www.cnet.com/tech/mobile/samsung-extends-android-and...
Access to cheap replacement batteries would definitely have an impact when deciding whether I should replace my phone or not. I believe Apple even mentioned the battery replacement program being a factor contributing to lower-than-expected sales a few years ago (sorry, can't find the source).
My previous phone lasted four years, and really I only replaced it because I have more money than sense and wanted a 120hz screen. iPhone 11 Pro, came out nearly five years ago, still totally fine, lots of them in circulation. The battery was certainly beginning to show its age by the end, but the phone was otherwise fine.
Your point was maybe valid a decade ago, but phones last much, much longer these days. And getting supported still longer; current phones, particularly on the high end, are far faster than they need to be to support the OS, and rate of hardware change has slowed significantly (Apple, say, hasn't had a full-blown microarch change in years, with the M1-4 and their sibling A-whatevers being very much incremental progress on each other).