Go check a place like AliExpress: plenty of those there.
It's not even as if dumbphones are amazingly complicated and highly dependent on complex software to work - the actual complex mobile network stuff comes inside modules that do most of the work.
If dumbphones aren't reaching people's hands in some countries the problem is in distribution or maybe lack or awareness: we do live in a Marketing-heavy society and people are almost conditioned to go for expensive branded stuff.