The reason is simply that phone calls across Europe were and are usually not free/included in flat rate plans. Telcos just never found a way to agree on something that makes sense: It's quite common to get unlimited domestic calls, but pay 10-30 cents per minute to a neighboring country.
Nobody (other than businesses) actually pays that; people just ended up using WhatsApp for both calls and texts instead.
So in a way it's the absence of regulation: Data roaming is free within the EU, thanks to a corresponding EU regulation; before that, it wasn't unusual to pay more than EUR 10 per Megabyte (yes, Megabyte, not Gigabyte).
Regulation 2018/1971 limits the price for outgoing calls within the EEA to 0.19 ct/minute (excl. VAT). Interestingly enough, there doesn't seem to be a limit on incoming calls, though most providers don't charge for incoming calls.
There's no charge for incoming calls from abroad while at home: All EU countries use "caller pays" billing (unlike the US, which historically has used shared cost for mobile phones).
That's why there are prepaid SIMs without a monthly fee and unlimited incoming calls: The caller's operator pays the called operator a termination charge by the minute.
There is an incoming call charge while roaming, the rationale being that the caller doesn't know where you are and needs predictability in pricing (so the called party pays for the leg from their home country to where they are), but the EU has capped that to zero within the EEA.
Nobody (other than businesses) actually pays that; people just ended up using WhatsApp for both calls and texts instead.
So in a way it's the absence of regulation: Data roaming is free within the EU, thanks to a corresponding EU regulation; before that, it wasn't unusual to pay more than EUR 10 per Megabyte (yes, Megabyte, not Gigabyte).