The theory, as I understand it, is that the wider ecosystem of OEMs is better at selling new hardware than Microsoft alone is at selling Windows upgrades. The users don't care what makes new hardware "new hardware", just that a dozen different companies are telling them that "new hardware" is exciting to buy for the holidays and "more secure" and "better". The TPM requirement on paper is an easy shibboleth for "more secure", so an easy thing to sell through the multi-channel telephone game of OEMs to ad companies to retail stores to mainstream zeitgeist. They don't have to just take Microsoft's word that Windows 11 is "better", they have "word on the street" and their pal who works "Geek Squad" at Best Buy and all those HP commercials on TV telling them they need a new Windows 11 machine for "more secure" hardware.

(I think it is gross that this is how Microsoft and the PC OEMs think is the best way to increase revenue together, but I think there's enough evidence that this theory is relatively accurate portrait of one of the factors for why Windows 11 is the way that it is.)