Part of the problem is that some teams that have the cap space don't have their own picks that are required to make an offer sheet (for example. the Devils don't own their 2021 2nd round pick). The flat cap has also impacted teams that otherwise may have considered an offer sheet - at the very least, the flat cap reduced the ability of contending (i.e. more desirable) teams from having the cap space to make an offer or unload other contracts to clear space.
Mostly though, there is probably little incentive at this stage for any of the RFA's to accept an offer sheet. They probably would all prefer to stay in Tampa, especially after being in a bubble for months with their teammates and winning a Cup. That has to bring a team really close together, so I doubt they want to screw over their team/teammates by accepting an offer sheet at this point. Regardless of how this offseason turns out, Tampa is also going to be competitive for the foreseeable future, so who wouldn't want to stay in that situation?
That doesn't mean that there hasn't been an offer sheet made or that the RFA's haven't had any discussions with other teams. For now, they have time and are likely hoping for Tampa to get under the cap somehow while giving them a fair contract.
If we get closer to the start of next season and these guys are still unsigned and contract negotiations are not going great, then they may reconsider if an offer sheet comes their way.
Bingo. An offer sheet being signed was either going to be right away in free agency if a guy didn’t want to stick around and wanted something long term (obviously not the case) or late in the game when it’s clear there isn’t another option.