From just the headline, I expected this question was going to lament the "wasteful" coding practices of mobile apps and pine of the days of hand-tuned, low footprint software. So, I was all set to point out that smartphones with multiple cores, GBs of RAM and storage, high speed networking, and massive screen resolutions are nothing like 80s computers. These things aren't even as limited as computers from the mid-2000s!
But, as others have mentioned in various posts, I think the answer to the question of BASIC REPLs has multiple angles.
First, most early 80s computers did not boot straight to a programming REPL either. As soon as you had some kind of disk operating system you ended up in a shell and would have to invoke a BASIC interpreter or other programming environment if you wanted it. As far as I can tell, the feature threshold was having enough storage to make multiple programs persistently available. That was true across CP/M, MS-DOS, and Apple computers. Then, when they added GUIs, the shell became some kind of GUI menu instead of a CLI. With more storage, you get more built-in apps and less emphasis on a "blank machine" ready to take custom code.
After decades of this growth of storage, a smartphone is really not marketed as a general purpose compute platform. It still has more than vestigial "communications appliance" characteristics and is morphing into "cloud appliance". There is a feedback loop where vendors are marketing an integrated experience that sets the expectation for the next round of products too. And at the mass market volumes they are reaching, economies of scale mean that this approach is targeted towards the largest consumer markets.
It's almost fractal, but the cloud applications themselves are going through the same kind of shift. The commercial pressures are to create ever more integrated experiences for the mass-market user. It is a niche interest to want a general purpose platform where lots of capabilities are available, but the integrated whole is absent and waiting for a new custom program to be entered. Most consumers don't want the device or app that lacks this complete solution, and vendors don't want to provide all the infrastructure and then have some other party come in and claim all the value-add experience that is most visible to the paying customers.