Is it so hard to use an adapter to USB-C?

These days wireless earbuds are probably more popular than wired for mobile use.

Adapters, like dongles on laptops, are an inconvenience. Bluetooth does not work well in general, provides only low quality audio, and even worse performance on calls trying to stream input and output. A significant amount of people (maybe a majority in the market for non-Android, non-iPhones) still use wired headphones because of these disadvantages to wireless.

Seems like a serious miss with the 4. Hopefully they'll bring the jack back on the 5, unless it's too expensive, hard to waterproof, courageous, or whatever the new excuse is for ditching it.

The Fairphone 4 is only IP54 so waterproofing is likely not an issue, I've never used the audio jack on my phone so I really don't mind.

Keeping the cost down presumably

It goes against repairability, and sustainability which they're trying to promote.

I was very optimistic about this phone, but it looks like I'm going to have to go if the Google 5a

The headphone jack is a strange hill to die on when you get the chance to replace pretty much every important part. A well made dongle does not make a massive difference when you use headphones with a proper cable (I.e. not the flimsy thin headphone cables that rip or break within a few months of regular use).

It's not strange at all -- using headphones while charging is a daily situation for some