I don't know why the reason is, but Monero scanning is very very fast in comparison. So far, any Silent Payment integration I've tried is impractically and painfully slow. I've given up using it for now. Maybe it will improve down the road?
Yea, its a UX trade-off for sure, especially vs other cryptocurrency, but a relatively simple one because all you have to do is open your wallet and wait. Nothing else. And if you use it somewhat regularly it's pretty damn quick to catch up. I think Cake Wallet is going to release periodic background syncing soon which should improve it UX once more wallets adopt this.