The problem is that Apple, Google, and Microsoft want to conflate "tight chains of custody for software distribution" with "we want to be a gatekeeper charging 30 percent on every sale." TBH, the number of dodgy (absurd subscriptions, me-too clones, toxic ads) apps on mobile platforms suggests they aren't even really capable of delivering that promise. If you really want to push your platform as "the place you can trust", especially for nontechnical people, you've got to go beyond signing to actual curation for content and quality.
Conversely, putting a huge financial and certification burden on developers hurts power users and enthusiasts-- how do you bootstrap a new programmer if he has to spend a week generating (and paying for) certificates and learning signing tools before he can emit his first "Hello World?"
Only for a certain group of users, while another group of users is degraded to guests on their own systems while "the cloud" ends up having the actual control.
Conversely, putting a huge financial and certification burden on developers hurts power users and enthusiasts-- how do you bootstrap a new programmer if he has to spend a week generating (and paying for) certificates and learning signing tools before he can emit his first "Hello World?"