I've hired a decent number of engineers for a few different companies, so I'll take a stab at this based on what I've seen.

- Fixed costs for hiring for PT and FT are the same, but the company gets more output for FT, so they focus on that

- Similarly, external recruiters who take a % of salary get paid much less for the same amount of work, so you're not worth it to them

- Fragile processes require lots of interaction. Having someone unavailable slows down the team

- PT employees are perceived as less committed. Enough ink has been spilled about companies wanting dedicated people, so I'll leave that one alone