Less the costs of transferring jobs.
The industry has done everything in its power to try and retain people through disincentives (interview processes, on boarding process, expectations of often absurd spinup to productivity times, etc.) while not focusing on the incentives (increased pay, more time off, better work life balance, etc.). All of this, acting as sort of collusion agreement proxy to reduce poaching across the industry, makes it less attractive to jump ship and businesses know that.
Spending the time to update my resume, market it a bit, deal with a mixture of mostly sweatshops with some reasonable employers mixed in, and a few actual good employers hiring processes (recruiter spam, LC gauntlets, full day technical interviews) and often employers expecting you to clairvoyantly know their idiosyncrasies (internal infrastructure, business processes, pre-existing code bases and dozens of arbitrary dependencies they chose and worked with for years)--all of which businesses pretend aren't costs and externalize on employees, makes it less attractive.
Let's add in the mix of normal/traditional job switches: meeting and meshing with new coworkers, changing your commute which may not be insignificant, overhead of switching and moving your benefits packages around, delays for building/taking time off, new uncertainties/risks about stability, etc. and suddenly, that sign-on bonus with the near same package doesn't look so attractive.
Anymore, for me to even consider jumping ship I'd need a pretty hefty sign-on bonus just to deal with the rest that will turn into lost personal time and stress. I really need a significant TC bump and hopefully, better work/life balance and/or a combination of my current employer TC degrading due to increased demand, hours, stress, stagnant growth, etc. otherwise, industry has created so many disincentive barriers that I'll stay where I am.
Maybe it's not as tiresome/costly for some, but it certainly is for me.