What Einstein did not know and did not incorporate in Spacetime is that Matter is moving outwardly in a straight line at the fastest speed we are involved in.

The Milky Way is moving in a straight line, so to speak, in a single direction, away from where it all began, and Einstein did not know this, originally.

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Mechanics to the rescue because two boats of the same size, moving parallel at the same speed, and relatively close will end up touching one another.

Are you paying attention?

These boats are moving parallel, not touching one another, nothing is changed and yet they still end up touching one another.

This will not work when the boats are too far apart, so proximity is important.

Mechanically, the wakes of the boats are involved. The wakes in between the two boats make the boats move toward one another.

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The Sun and Mercury are not of the same size, but they are like two boats moving like-parallel in this fastest direction (their gravity is also in play, so we do have a complex situation on our hands).

It is like an enormous boat and a dinghy. The wake of the large boat is enormous, but the dinghy just far away enough to experience it without being pulled in.

Naturally, analogies are to some extent always imperfect, but this should help you see how mechanics are involved.

Einstein left out the most important Momentum there is in science. He did not incorporate the most important motion in science.

But Einstein was smart enough to understand there was an additional framework at play. He just did not know about this fastest Momentum. He therefore figured out what was needed to explain Mercury's behavior, and hung that hat on Spacetime.

My guess is that Einstein either did figure it out later and kept quiet about it (it would remove his name from history) or that he did indeed not see this fastest speed and how it explained all, the old-fashioned way, until the day he died.