Stevens and especially Brissett are probably both in the top tier of prime-ages guys who aren't on a roster right now. But there just aren't any roster spots available. Neither are as good as Lonnie Walker, IMO, and he couldn't get an NBA job, either.
It seems like as older vets tend to play longer and longer, and CBA changes have made the 15th roster spot all but imaginary for cost/apron/trade flexibility reasons, at least until after the March 1 buy out deadline, borderline mid-to-late 20s guys are mostly who are getting squeezed out.
Every team still carries at least a couple recent-draft-pick developmental guys, and often 4-5, and more teams now have a late-30s veteran in their rotation than in the past, so there just isn't room on effectively-14-man rosters for guys like Stevens and Brissett.
I could see one or both catching on somewhere late in the year, but there are still cap/tax advantages to converting 2-way guys instead (rookie-minimum is significantly less than normal vet-min), and the new CBA rules now allow you to sign players to new 2-way deals up until March 4th, so teams can convert a 2-way player to a standard contract after the March 1 waiver deadline for buy-out guys, and still sign another young player to a 2-way for the rest of the season (and beyond). That further limits the roster opportunities for guys who have more than 3 years of NBA service and aren't eligible for a 2-way, like Stevens, Brissett, and LW4.