Being a subsistence farmer was incredibly risky. One bad year and your whole family starves to death. People only did that because there was no alternative. As soon as the industrial revolution came, people left their "self employment" en masse to work at a company.
Not even "work at a company". Most of the dairy farmers here where I'm from are part of a collective called Arla where the independent dairy farmers collaborate, effectively building their own safety net with an organization that could support them if they had a bad year.
Companies are not required, but social safety nets are hugely important for modern systems of production.
At least in the case of England, it wasn't initially voluntary. The enclosure acts removed their ability to farm and feed themselves, removing the last benefit they received from feudalism. It was from lack of means to farm anymore they moved to the city.
