The gap between an accepted offer and a first morning carries several obligations with their own timing, and most are far easier to meet before anybody arrives.
Hiring a first employee starts a sequence of federal, state and local registrations, several of which have to be complete before anybody can lawfully be paid.
Most of the damage from a price increase comes from how it was communicated rather than from the number, and the sequence that avoids it is well established.
The cost of losing an employee lands as lost capacity spread across several months rather than as a recruiting invoice, and the causes are mostly week to week.
An introductory period is not a legal status across most of the country, which means whatever value it has comes entirely from the structure put inside it.
Most underpricing comes from a small set of untested assumptions about how customers choose, and each one is answerable from what buyers do rather than what they report.