Because that requires investment. If they make that investment and are wildly successful at taking market share from x86 then ARM will become a much more valuable company. Buying ARM allows Nvidia to capture that value.

This is a real problem when some of that investment is porting/tuning various open source projects to make them run well on ARM. Such work would benefit the whole ARM ecosystem, including competitors who sell ARM based chips. At which point, owning ARM starts to look sensible.

Having an ARM license has zero relationship with performance.

An ARM license is only for unit cost benefits. Nothing else.

The Tegra's aren't that bad are they? The CPU in the Xavier seems pretty decent, and it's only on a 12nm process. If they can get that shrunk down, it'd probably run even better.