As the quip goes, 'the President proposes and Congress disposes'

In the sense that the President (and the executive branch behind him) almost always have greater unity of purpose (it should! The President technically picked them!) and therefore clarity of vision than Congress.

But Congress (as a largely power-distributed, multi-state-representative body) is ultimately empowered to say yes or no.

As a realpolitik result, Congress (of same or opposition party) generally lets the President lead with a proposal, and then adjusts it to their satisfaction into something that can pass.

Congress would likely be realistically unable to generate a yearly budget, if left to their own devices, due to competing interests and priorities.

Having a starter plan from the President cuts through some of that, even if they strike large sections of it.