Not true, modern BT codecs can be essentially transparent, and cheap 3.5mm jacks can have all sorts of issues. Bad noise performance, bad interchannel isolation, pop click issues, etc. don’t underestimate how badly you can design a 3.5mm jack especially if the phone only costs like $50 total.

Despite what audiophiles believe, DACs are a relatively solved problem and quality does not correlate with price.

As a non audiophile who designs this stuff for a living, you’re correct it’s not difficult or even particularly expensive. But it’s also surprisingly common to do it poorly.

The only truly, audibly lousy Bluetooth codec is low-bitrate SBC. Anything better than that is going to be transparent to most ears.

The awfulness of cheap/low-power DACs, on the other hand, knows no limit.

There is going to be a DAC somewhere, and somehow putting it in the headphones doesn't speak for quality. Power budget, if nothing else.

It’s much easier to make a headphone output that is tailored to a very specific driver than it is to make a generic headphone driver that can drive any conceivable headphone plugged in.