Relatively few engineers actually want to do contract work. The day rates look high, but they have to pay self-employment taxes and health insurance, as well as charge more to cover inevitable gaps between contracts. It seems unlikely that the existence of contract work is preventing senior engineers from joining your company. To be blunt, it’s more likely that you’re simply not paying enough to be competitive or you…
Most SWEs doing 'contract work' aren't actually 1099 contractors, what it means is they work for an intermediary and then 'contracted out' to a big company with a recognizable name (Google/BofA/Verizon/IBM). These jobs are W2 so you don't pay self-employment tax, they usually have mediocre insurance options, and you lose your job as soon as the work dries up.
It's simply a way for a company to staff up quickly without going through as strict of hiring committee and having any obligation to keep said staff around if plans change.
It's often easier for a department to get a req for a senior/staff contractor (and make salary exceptions) than a FTE, so what happens is departments will offer 200, 250 an hour for temp work but only able to hire engineers for 150k a year because their 'employee' req is slotted lower.