For large companies, division managers are more concerned about short term goals, because that's usually what metrics are in place, but also potentially competitive landscape, etc.
Bringing in someone today who can get something done in, say, 4 months, vs going through a hiring process, or fighting to reprioritize existing projects/staff... that's a big headache. Throw a bit more money - often from a different budget than "payroll" - and 'solve' the problem.
I get your point, and am largely sympathetic to it. However, often, the people with the required skills don't want to work 24/7 for one singular employer. You lose a lot of freedom/flexibility, for starters. Often a pay cut vs 'consulting' rates. I've been out of traditional employment for so long, I'd probably not make a good employee anymore, and would have a hard time getting hired.
As an external consultant/expert on some things, you're also being paid for explicitly not being part of the 'problem'. You can ignore some of the politics that may have led to the situation you're trying to fix. The person cutting the check may be able to cut through some of the red tape and get stuff done quickly.
I've been an employee at places where it's taken a couple days just to get myself on a network and credentialed up. That's a pretty significant waste, but... amortized over the total employee cost, it doesn't seem to be seen as that big a deal.