Just another instance of common anti-consumer behavior from multi billion dollar companies who have no respect for the customers that line their pockets.

They used to dominate the consumer market prior to Ryzen so might have something to do with it but I got no evidence lol

Intel also sells the chipset and the license to the chipset software; the more boards get sold, the more money they make (as well as their motherboard partners, who also get to sell more, which encourages more manufacturers to make Intel boards and not AMD)

There are many motherboard manufactors but only 2 CPU manufacturers (for PC desktop). Board makers don't "makes deals" so much as have the terms dictated to them. Even graphics card manufacturers made them their bitch back when multi-GPU was a thing - it was them who had to sell their Crossfire/SLL technology on their motherboards.

guess who sells the chipsets to the motherboard manufacturers

They had integrated the L2 on-die before that already with the Pentium Pro on Socket 8. IIRC the problem was the yields were exceptionally low on those Pentium Pros and it was specifically the cache failing. So every chip that had bad cache they had to discard or bin it as a lower spec part. The slot and SECC form factor allowed them to use separate silicon on a larger node by having the cache still be on-package (the SECC board) instead of on-die.

AMD followed suit for the memory bandwidth part from the K62 architecture. Intel had no reason to do so.