I'd guess the hardware is the problem. Most antennas I've seen on access points are larger than the entire Pi Zero W. While I'm no radio expert, I'd expect APs to use the smaller antennas if they were capable of being as good as bigger antennas.
Ah, that reminds me of an article someone here linked to a few years ago: [1]. That claims the efficiency is -3.5 dB compared to a typical -1.25 dB dipole in dedicated wireless equipment. Not bad; the difference is only a factor of ~2 in power for x8 reduction in size.
