My first reaction was to say that's wrong. Maybe it's just new, but last time I checked, a couple of years ago, there's no way to get a decent land line under 40€. The trick was that the "cuota de línea" is hidden from the listed price. Most of the land lines are owned by Movistar that rents them to Vodafone thieves and other ISPs that consider it an independent expense, as if it's not just a regular cost. Another possibility are time-limited offers.

Now it's my landlord who pays so I don't care, but still would like to know where did you find that price.

You just need to negotiate with them, not just accept the first base offer.

https://i.imgur.com/kHDdXZ1.png

how/what are most people using internet for that they tolerate such small caps?!

If you have no WiFi access it's a somewhat different story, but WiFi is ubiquitous.

It's also cheaper, a 4G contract with a 100GB cap is about 15€ per month, whereas my FTTH connection at 900Mbps down / 400Mbps up cost me 40€ per month.

In Lithuania, i paid 15 EUR a month for 500/500 and it worked perfectly too.

U.S. prices are super high - mostly a consequence of most people living in detached houses - long distances and high complexity of wiring. In Russia and Lithuania, most live in Soviet-style apartment blocks - high user density.