In my 6 years experience I've never seen this either. Here in Germany maybe it's a little different - contractors are very common at large companies, because it can be very hard to fire people once they've passed their trial period (3-6 months). So companies can be more reluctant to make permanent hires. Another reason is that a department might have a strict limit on their head-count (of internals), but not their budget, so they can still increase their workforce by hiring contractors (don't ask me why, just big company politics).

At the two large companies I worked for, around half the team members were external. In both cases there were 3 types of worker:

1. Internal

2. External contractor & a permanent employee of a contracting ("body-leasing") company

3. External contractor & a freelancer / self-employed

In terms of the tasks and technical responsibilities you have in these roles, I didn't notice much difference. The main difference in case 3 is that you get significantly more money (like 1.5-2x more), in exchange for worse job security. Given how easy jobs are to find in the current climate, I don't understand why more people don't opt for this. In case 2 in particular, the contracting company is taking up to a 50% cut of the hourly-rate, it's kinda crazy.