I've replaced more batteries (and screens) than I can count, and it's increasingly difficult and complicated. 5 years ago or so I'd agree with you, but now there's no phone I can easily open without heat gun, controlling the air so no spec of dust land on the lenses (and a blower to remove in case it happens), and almost always I need adhesive (B7000) to patch or replace the original one to keep similar level of weather proofing. It's easy if you pay 100 bucks someone else to do it, sure.
Back in the days of my HTC Desire I could carry an extra battery, or two, in the pocket, without issue. Nowadays I'm married to a power bank that needs to be plugged for the duration.
That's cheap. If you think that a safe first-party replacement battery will sell for less than the 79€ that the whole replacement effort takes, then you're fooling yourself.
I strongly suspect that there's also not good language for blocking against third-party batteries (and the phone manufacturers would have good reason to do so because it might result in overheating or worse with really bad third-party batteries).