The question is: Why do they have to?
The answer: it's a game of chicken.
Here's the situation in brief: The old rules for labor/management relations (CBA) have been voided by the owners. The owners hated the old rules because they thought they didn't make enough money.
At the end of this year, the owners will likely lock out the players. That is to say, they won't let them come to work. They want to do this to force the players into a bad position financially where they have to settle for a bad deal with the owners.
The players' union has set up a provision to decertify (cease to become a union). They haven't taken this step yet, but they are ready to, if talks break down. If there is no union, the owners can't lock out the players. It would be illegal. The players would show up to work with all their current contracts intact. It's not a good long term solution, but it would force the owners' hand in the sort run.
The owners don't want to play by the old rules any more, so they have to do something to make the players not want to play under the old rules either.
This is where the 18 game season comes in. A provision in the CBA gives the Commissioner the unilateral right to establish a season of up to 20 total games. He doesn't need the players approval for it.
However, in a NEW CBA (not yet negotiated), the players would have to approve any and all changes. So, today the owners presented the player an 18 game season plan. They don't have to do so UNLESS there is a new CBA signed (which is what the OWNERS want). The 18 game season is an sword suspended over the head of the players. If they choose not to negotiate, break up the union and show up to work next year under old rules, they'll have to play 18 games for the same money as before.
They obviously don't want to do more work for the same money.
By offering the players an 18 game proposal, the owners are screaming, "WE WANT A NEW CBA!". The players are now over a barrel. If you don't think they'd like 18 games for more money, they sure won't like 18 games for the same money.
If there is football next year, it's going to require a new agreement.
This is turning into a game of who will blink first.
I'm not sure who the chicken is, but it's the fans who likely be fried.