Because it's been made intentionally difficult, with significant risk of further damaging the product you're trying to repair. And there's no official supply of replacements.
A battery change shouldn't require heat guns, special suction/clamp tools, or adhesive-dissolving chemicals.
It also shouldn't require being without your device for an unknown period of time to send it in for service.
Reading through bluescrn's comments, it seems like anything short of "I can easily replace the battery myself" is unacceptable. Would I like to be able to replace the battery in my Apple devices myself? Absolutely. Is it an unreasonable amount of money and effort to have the Apple Store do it? Not at all. Getting them to replace the battery is no big deal. If there's something to complain about, it's that they (probably intentionally) don't do a good job advertising their battery service.
A battery change shouldn't require heat guns, special suction/clamp tools, or adhesive-dissolving chemicals.
It also shouldn't require being without your device for an unknown period of time to send it in for service.