The last house I shared with roommates had myself and four other people. The bedroom was just big enough for a bed, desk, dresser and an office chair. We all go along, but that many people in a shared space and you are always tripping over each other and all up in each other's business due to the confines of the house.
Kinda hard to get romantic with a partner of choice when there are four other people watching your every move and judging the new stranger now in the house. People I know have then had the issue where bringing a new person around, even if just a night or two a week changes the dynamic in the living space. If you have a good place and decent roommates, most people will not want to disturb that, especially if rent is outrageous and/or finding a new apartment/room/house is difficult.
I'd lean more onto the 1.7 trillion dollar Student Loan problem combined with the massive out of control housing costs in the areas people want to live in. Add in working long hours/multiple jobs as a consequence and you also explain why people under 30 are not joining professional societies and hobby organizations as much as their parents did. (And not joining and taking part in those activities limits contact with new people, reducing potential romantic partners) Work 70 hours a week and you won't have the energy to do much else.