You're not thinking in terms of performance-per-watt-per area. Intel could've gone with larger cores (albeit less of them) for a given area, but more large cores would consume more power in the same area. So instead Intel chose to maximize this area with a blend of high performance and low-performance cores.
If the windows and linux kernel thread schedulers can take full advantage of the two types of cores and efficiently schedule threads, then you get good performance efficiency.
Case in point: if the rumors are true, the PL2 of the 16-core 12900K is 228W, which is less than that of the 8-core Rocket Lake. If (and this is a BIG IF) the chips really do perform well, and within this PL2 power budget, then Intel would've pulled off quite a feat! More cores more performance, and more efficiency. That's good engineering.
Just because a desktop CPU is connected to a beefy power supply doesn't mean the CPU manufacturer(s) shouldn't try to be as efficient as possible. The future of CPU design is going to be tiles/chiplets and blend of different types of compute. We just have to see how the new cores perform in Windows/Linux... to know whether the experiment is worth it or not.
But don't fault Intel for being an empiricist and testing how to breathe life into x86 with new designs. The ARM attack is coming. Apple M2, Qualcomm Nuvia, Ampere Altra, possibly even Nvidia. Intel and AMD have to innovate especially in terms of efficiency to blunt the attack that is coming. Customers want more efficient cores. And Intel is trying to deliver that to the ecosystem.
And besides, this will allow Intel to simplify and unify its cpu-design. They can have one design for both laptop and desktop... unlike 11th gen which has different core designs for both laptop and desktop. If you look at Apple for instance, Apple is able to manufacture one chip, the M1, and put it into many different products: Mac mini, MacBook, iMac, iPad. Much more simple (from a design perspective) than having to expend energy to design, test, and manufacture multiple designs for desktop/laptop/tablet.