You sound like you're very young and only joined mobile internet in the past year, but I'll bite: not long ago pretty much all mobile internet was metered, that is, you had for example a 1 GB / month rate limit, after which every 100MB would cost you additional money on your already expensive monthly subscription.

So people were careful with using mobile internet, there were stories about people who forgot to turn off mobile internet and had to pay hundreds extra, or people who used a lot of mobile internet by accident abroad, racking up bills in the thousands. Apps that had bigger downloads would often be on the arse end of complaints about high data rates, so it became a UX pattern to warn or disallow big downloads over mobile internet.

4G / higher bandwidth and government legislation (the latter notably in the EU) finally put a stop on that, only two odd years ago (if that). But, there's still plenty of countries without 4G, good internet backbones, and who still do metered connections. Even wired connections, notably in the US which seems years behind in terms of internet infrastructure in some areas.