Earlier quoted context omitted.
In most cases this is because 1) they're not ergonomic to use and 2) you can create layers to access the function keys.
I know that, but layer (to me) make harder to do stuff, specially if need to do some key bindings that require more than 1 key (like ALTS, TAB, etc). This push to do heavy customizations but that create the issue of work only for X machine and X OS. In my case, a do work inside VMs and customers machines by remote software and also need to work in other laptop, so I prefer to not do keymaps customizations at all...
That said, I think it's a good idea to keep function keys if you use them. There are some keyboards with them, but it's surprising uncommon.