Because the service provider receives the unencrypted email and can choose to save a copy, encrypt it to a different key, etc. This was the scam Lavabit pulled, and the government called them on their bluff and asked for a copy of the key and Lavabit had no legal ability to refuse.

If the threat model does not include a government with the ability to use legal process, it needs to be defined more precisely. In general the US government can use legal process in the US and just straight-up hack into things elsewhere (who's going to raise a diplomatic incident over it? Russia is literally poisoning people, nobody cares, and their military is less powerful than the US's). If your threat model is other governments or just unrelated attackers like advertisers, there are more straightforward approaches.