A ridiculous reason, such as being able to interact with the federated world from my own server directly, and not through a third-party service. Why is the simple concept of going to your own server to interact with the entire network, getting content directly from your friends' servers, and receiving interactions (e.g. PRs) directly from someone else's server such an afterthought in ATProto?
You can host your own data very cheaply. If you don't have to serve every single other server, and keep them all up to date, and can just pump some records out at aggregating relays, it makes it much easier for people to be online. You can run a massive PDS on a tiny ancient rpi.
Which is what activitypub doesn't do. It's protocols are very heavy and hard and stateful. Running mastadon or other servers requires peering with every other related server. And it's on you to deliver messages, to push, to track state and progress. All the network work is on you to get your message out. So your server with your data has a ton of work to do. This doesn't scale well. See again Paul's scaling with open social.
If you do want to run your own atproto services, yes, you need to pull from the network to see what is relevant, you need a relay server, you need a pull. And you can! Running those relays is very cheap! The protocols scale really well & is super well optimized to be efficient to do! You can just do it!
The one particular thing you keep trying to demand is what makes AP so hard, so difficult, and scale so very badly. It's the worst part of activitypub, it makes it super hard and super expensive. Switching from AP push model to the relay doing pull model is the secret sauce that makes everything super lightweight and scalable, and that makes sync so much easier.