Two main reasons:

1. Compute. It's easy to make a voice assistant for a few people. But it takes a hell of a lot of GPU to serve millions.

2. Guard Rails. All of those assistants have the ability to affect the real world. With Alexa you can close a garage or turn on the stove. It would be real bad if you told it to close the garage as you went to bed for the night and instead it turned on the stove and burned down the house while you slept. So you need so really strong guard rails for those popular assistants.

3 And a bonus reason: Money. Voice assistants aren't all the profitable. There isn't a lot of money in "what time is it" and "what's the weather". :)