There's more than that, still; high frequency is still a bit more expensive, but it depends. Mainly when you need high power density, so smaller capacitors and inductors can be used -- but higher quality parts are required, too.
So, cheap commodity units might be made at low frequencies, with old controllers (TL494 is still around..), and not be particularly compact, but they don't care. Or... they skip the EMI filter, because... they don't care.
The ultimate capability of Si MOSFETs are more of a limiting factor, one among many. You can use them up to some MHz with high efficiency, 100s of MHz even, but it's increasingly difficult (limited topologies, supply/load ranges, expensive RF types?). You're probably not going to go that high for a general purpose power supply, let alone one that needs to sell competitively in large quantity.
Engineering is an often multidisciplinary art, that aims to find solutions to problems that contain many, often poorly defined, constraints. Cost, size, electrical ratings, environmental and performance ratings, etc.
The constraints listed above are merely among them; they might not even be relevant when other constraints dominate the problem.
Tim