They do keep them for some time. Think of how desktop motherboards still come with PS/2 ports even though 99% of mice and keyboards are USB. And with laptops, many new ones still come with a couple of USB A ports and an RJ-45.

However, as with everything in tech, some things have to get phased out at some point. From a manufacturer's standpoint it's probably 1) cost saving measures and 2) making space for new things (because you can't keep all the ports that have been there historically).