Water ingress isn’t the issue & there’s been waterproofed ports for decades. They wanted to make devices thinner—but what value is it when its too thin to support a jack & made of materials that now require a case?
That's probably just the marketing reason. The realistic reason is probably that they want to sell you their brand of wireless earbuds that need to be replaced in a few years tops
Which is only likely to last one year anyways. After which, you can pay an exorbitant amount to replace the degrading glue. I'd just like my wired headphones back, the jack will last longer than a year at the very least.
People are going to try to make up reasons, all of them are an excuse to cover up the real reason which is copying apple and forcing people into Bluetooth to sell more expensive tech junk to people. The original and really only reason.