You don't buy a CPU alone. You buy a whole server. If the CPU increases 1000$ in price to gain 10% performance but the complete server already is 20000$, it's quite worth it to go from 20000$ to 21000$ for 10% of performance.

Also if you factor in a performance critical software for 100000$ that runs on that single node, you will buy the fastest hardware that you can get for a few thousand dollars more.