I’ll answer the headline question…
When you work full time for a company, they pay for private medical insurance, sick days, paid holiday, the hardware and software you use, the place you work at, the electricity, the support staff (HR) etc. Depending on the employment laws, they may need to pay a severance package if they want to get rid of you and it may take time to do so.
When you work freelance, YOU pay for any private medical insurance, sick days, holiday, hardware & software etc, etc. Your contract with them can probably be cancelled at very short notice as well so they have a lot less responsibilities towards you as a freelance worker. Also bear in mind that you might be waiting months for the next contract to start so you should definitely ask for more than what you’d get working full time.
It may seem like you’re paying more for a freelance worker but it can often be the opposite when you consider the additional cost to employ them full time. I often hear the average cost to employ someone full time as being approximately 10K per month though I believe that’s a US figure so that’s going to differ depending on where you live.