TIL "garden leave", a period of time when you are mandated to be out of work. Thanks.
Long enough to reset, not worry too much about health coverage, and if timed right.. not miss out on a bonus cycle. It's also short enough that you can interview and get a job offer from a company willing to wait for you. Often you can negotiate a signing bonus at the new shop to make up for compensation you may lose due to deferral or bonus cycle.
The firms trying to force 18 month terms are also some of the highest turnover shops. Arguably you'd need to demand a 50-75% bump to take the role since on the way out the door you are going to miss out on 1-2 bonus cycles. The term is so long you likely need to quit before finding a new job too, and then kick off interviewing in the last 6 months.