H1 Transfer - joining on receipt (layoff)

Here is a situation:
I work in Company A
Got an offer from Company B, and they started the H1B petition process and have filed the petition.
Company A is going to undergo layoffs in the coming weeks.
hence I want to start at company B upon receipt, but company B is repeating the same stuff about best practice to join with approval which will take an 15 business days (3 weeks) and I may get laid off before that.
If I am already out of job, why shouldn’t I be able to start at company B ?
What are the legal implications on company B ?

What arguments can I make to company B to allow me to start immediately ?