I think you probably know the answer already, but...
It's because Celtics management is already bracing for a future without Jaylen Brown. The two most likely scenarios are that A. The Celtics can find a trade partner this season that will give up the exorbitant amount Danny Ainge has repeatedly asked for in return for Brown (think about how unlikely this is after what they've already passed up) or B. Free agency comes along and another team with cap space and a long-term outlook offers Brown a deal that the Celtics can't/won't match because of cap implications (most likely scenario IMO).
Some people really believe the Celtics are going to go into the luxury tax to resign Brown next year, but I just can't see them doing that for a guy who was benched last season in favor of a hobbled and emotionally fragile Gordon Hayward. Hayward is taking up more than a quarter of the Celtics' salary cap for the next two seasons and they've shown an obvious affinity for him over Brown.
A lot of what you're saying here isn't correct. First, Gordon Hayward and Jaylen Brown were both starters to begin the season and then both were 'benched' in favor of Smart/Morris. Hayward never took a starting spot from Brown. In fact, they both played very well with each other off the bench and that's likely a reason they will both start this season.
Also, the Cs are just not going to let Jaylen go for nothing. Right now, he might be the premier FA target next off-season (yes, it's a weak class). If the Cs and Jaylen don't come to terms on an extension this summer, then the Cs will have the option of giving him a max contract next off-season, matching a max offer sheet from another team, or threatening to match an offer sheet and doing a s&t with said team where the Cs receive a number of 'assets' in return.
Romeo Langford may prove to be the real deal, but there is like a .01% chance the Cs simply let Jaylen walk in RFA without receiving anything in return.