I wonder why more of the larger companies don't just do this themselves.

When I got out of college ~13 years ago, I went to work for a large retail corp. Their stuff was all mainframe and COBOL. They had a hard time finding new hires obviously so they had their own internal training program. The interview process was intense, around 4 hours of face to face interviews followed by a 3 hour written test (language independent, all logic and flow charting). If you got accepted, you were brought on and paid a salary for 3 months to go through their mainframe training program. The total cost for them was around 15k per student, which is far less than what I see the big tech companies offering as a signing bonus.