Why is adding #Signal to a new PC and to a new phone such hugely different experiences?

I can just install the app on a PC, open it, log in, and get all my messages over there in an instance.

On a phone, on the other hand, I have to switch over my account, so it stops working on the first phone, and then have to go trough an extremely slow transfer of my old messages.

I understand most people only have one phone, but the difference seems weird, no? Especially the difference in message sync