You’re probably in for a nasty surprise when you look up the process for replacing the battery pack in an EV, which is a more apt comparison than the standard hunk of lead and acid in an ICE car. Just like Apples laptops, EVs are trying to cram batteries into every possible space to maximize capacity, meaning that replacement will involve some substantial disassembly.
And yes, Apple can do it and has even said they’re going to do it. So at this point, it’s just a matter of guessing for why they don’t do it now. And that could be any number of reasons, including easier manufacturing or in a surprise twist maybe easier repair. Look at the ifixit instructions and look at the spacing available to access the pull tabs for the adhesive around the various frame parts. It’s entirely reasonable to think that Apple did some math on battery repair frequency; on that fact that even when an M* systems battery is at the 80% mark where it’s eligible for repair that it will still be near or over 100% of the old intel laptop runtimes when new; evaluated the chances of those adhesive tabs breaking or having the battery or top case damaged while removing one or the other and decided that bundling the two together and eating the cost on battery repairs was the easier option for their repair people. Because ultimately Apple’s priorities likely are ease of manufacture first, ease of store based repair second and ease of user based repair third. Because even with really easy user based repair, I would venture to say close to 90% of people will still have Apple do it for $200. And I say that based both on the fact that Apple used to offer various DIY repair options for older models* and people have their mechanic or AutoZone replace their car batteries for them even though that’s literally just 2 bolts.
* I worked for an Apple authorized repair place in the G5 iMac days, in which for example the power supply was “user replaceable” and for which Apple offered customers DIY repair options on them. I can count on one hand, with fingers to spare, the number of customers who took us up on the option to order the part and do it themselves, even if having us do it meant longer turn around times. On the whole people don’t DIY repairs for things, and so engineering and manufacturing for that use case is probably a worse decision than engineering and designing for your own people and while those people might benefit from DIY friendly features, if that makes manufacture harder or more expensive, then that’s still likely a net negative trade off.