Probably because there's effectively not a single person who explicitly _doesn't_ want their RAM soldered on to the mobo, and soldering on the RAM allows them to make the product thinner, which people _do_ want (or at least can be convinced they want through advertising).

If the amount of extra people who will be convinced to buy a Mac because it's thinner outweighs the people who otherwise would have bought one but didn't because it has soldered-on ram, they're gonna do it.

Because it is cheaper and 98% of the users don't think about it and don't care.