Majors also hire a lot of military pilots, and those are not eligible to upgrade right away, because they can only credit 500 hours towards the 1000 hours of experience credit required to upgrade by FARs.
My current airline only has the requirement that captains must have the FAR minimums to upgrade plus 2500 hours of total time. If someone gets hired with a restricted ATP At 1000 hours total time, then they cannot upgrade when they reach 2000 hours (1000 hours of 121 time plus 1000 hours previous time). This restriction is due to insurance costs.