This is only because the telephone cartels control the networks. The same is more or less true of Internet. Operators have advocated for anti-open-wifi laws across the globe so they can sell their internet access plans (xDSL/3G), when we could have free networking for all in all places.
Seriously though, why couldn't we have FREE privacy-friendly networking as a public service?
Here you have to buy access to certain frequency ranges(big money required). The bidding happens from time to time. Building the network infrastructure is not cheap either and requires specialists to work for you.
I'd rather buy that 4G from a company that can deliver it everywhere I go than move backwards in progress to use some random WIFI hosted by Joe that fights over the same frequency as John's across the road.
The "cartel" networks work pretty good tbh. They just have stagnated in everything else but networking.
Then you have to worry about fraud, and whether telcos in <random country> have been hacked, or are corrupt, and are leaking messages to bad guys.
I've seen entire (small) countries drop out, too. Usually the way you find out about this is that support notices an uptick in users in <smallish country> complaining about not getting messages, or you notice that the entirety of some geography isn't successfully completing the transactions you tried to protect with SMS. Er, you did consult the (changing) prefix database and phone number parser to (semi) reliably determine a geography from a phone number, right? Isn't parsing phone numbers fun?
It's fractally terrible and expensive, and I haven't even talked about APIs yet.