People, especially women, have other things they can spend time on instead of giving birth to 3+ kids.
I agree with you that other factors, such as cheap international travel and constant bombardment of people's highlights on social media, make it less attractive to invest in having a family.
Still, housing is a huge problem for Western Mediterranean Europe (Spain, Portugal, and Italy) and most of Latin America (Mexico, Brazil, Colombia).
Also for the record, in the US the birth rate per woman was below 2.1 for about 50 years.
So this is not really a new problem, it's just that there were enough young people back in the day and people didn't have the same life expectancy.