Designing in-house products is what they already do, and have done prior to Pop! so this shouldn't take user level app developer time that could be spent on better support for their users.
Building yet another distro is a waste of time and effort and only undermines goodwill with their users (I bought a Galago Pro to run Arch, and Pop came out 6 months after I received it -- I don't want yet another vendor controlled os). When Pop came out, they retargeted efforts around supporting only that system, leaving user choice by the wayside. The end result is that it burned me in the form of a missed firmware update that killed my battery, and customer support responses that boiled down to "run Pop!".
It's one thing to want to do something crazy -- it's quite another to hurt your customers in the process.