Apple aren't stuffing a third party CPU into an inadequate cooling solution with the iPhone because there isn't one. If there was an equivalent to i7/i9 in terms of mindshare in the mobile market I wouldn't be suprised if Apple released a phone with a low clocked and badly cooled one of those too.
Some releases have been worse than others, this would be my favourite example: https://www.notebookcheck.net/Apple-MacBook-Pro-15-Core-i9-s...
It might have escaped your notice that (1) third party ARM CPUs for mobile devices are not exactly difficult to find and (2) Apple was in fact using one before they decided to take the design in-house.
See also: Intel labeling middling 2c/4t cpus as 'i7' a few generations ago despite them not being at all comparable to the desktop equivalents because they know people will buy them based off the model number and not the actual performance. (https://ark.intel.com/content/www/us/en/ark/products/95451/i...)
And NVidia giving all their mobile GPUs the same names as the desktop cards (pre 10XX gen, they're actually the same cards with slightly lower clocks now) despite them being entirely different hardware. (https://en.wikipedia.org/wiki/GeForce_900_series#Products)
Actual performance doesn't sell nearly as well as perceived performance.
(Also I guess it's all about marketing for everyone)
I think I saw a bit of that phenomenon with Snapdragon variations and # of cores.