Surprisingly few. New grad training programs still depend on the "grad" part.
It's not like Fortune 500 businesses are stupid. If they could hire random high school grads for all their jobs and pay them less, don't you think they would? As you point out, college degree requirements have not always existed, they were added. Chesterton's Fence is a useful mental guide here. People don't generally make things harder on themselves for no reason.
Many many jobs today that require a college degree used to require a high school diploma. Yes those require intelligence and studiousness in varying degrees, but these skills can be assessed by interviews and hiring people in entry level positions for 4 years (or less!), not making them sit through irrelet college classes.
Half of a US college degree curriculum (the liberal arts distribution) isn't part of the curriculum at all in the UK for a science degree; are those UK degrees producing useless employees?
It's not like Fortune 500 businesses are stupid. If they could hire random high school grads for all their jobs and pay them less, don't you think they would? As you point out, college degree requirements have not always existed, they were added. Chesterton's Fence is a useful mental guide here. People don't generally make things harder on themselves for no reason.