There's all kinds of things they could add to phones - bigger battery, headphone jack, sd card slot, heart rate sensor, infrared remote, and loads more, and then all the hundreds of components they do include.
They have to pack it all into this tiny tiny amount of space and they have to make decisions about what to include. That used to include headphone jack, now it doesn't. You're upset about that, while I'd really love if phones included heart rate sensors. But we're both in a minority it seems.
It's important to remember that when you see lots of people on the internet complain about something, that doesn't mean that the majority care.
The majority don't care even a tiny bit so they don't speak about it at all. Only the minority who do care speak up. And if the majority of headphone jacks go unused maybe it's not reasonable for the remaining few who care to demand they stay - they are another potential point of failure and ingress in a phone, after all.