There's a huge shortage of professionals right now, so consulting firms are resorting to freelancers. While there might be a component of varible cost management in there, they do it mostly because they need someone to step in quickly to fill in a hole in a project (so they will try to get someone with the right industry experience and that doesn't need a lot of training). Recruiting could take longer to fill in the vacancy (if not from something else, because they need to test a bit more to be sure they want a long term commitment with you). 

Using a freelancer also means they can pay more to lure someone to that specific role, other than having to raise everyone's salaries on a permanent basis.

Of course consulting firms may be wary of the current economic situation - with COVID there was a lot of stop and go, and the current economic environment is far from being stable.

Does it make sense to leave consulting for this? That is really a personal judgement. They will treat you as a 100% team member for the project. They will respect you as they do on their employees. I don't think there's a risk on that. 

But you also ask for a few things that just don't make any sense - of course you are not eligible for advancement, promotions, bonuses, perks, paid vacation, 401k, etc. You are a freelancer being paid on a daily basis and that is your total compensation. If you want to “progress”, get a bonus or a raise… you do a great job and ask for a higher fee next time (or they may even just do you a job offer when you are about to end your cooperation - it happened to me). If you want time off… you have to ask, but it won't be PAID time off… or you take as much time as you want after taking on the next freelance gig…

I don't think there's any trick - you just have to understand you become a INDEPENDENT professional, along with the risks and benefits that come with that.