Check out "The Innovator's Dilemma" by Christensen. Innovation doesn't come from established corporations. It comes from startups.
Out of curiosity, what criteria are you using for "top companies"? Looking at the top 10 of the Fortune 500 (http://fortune.com/fortune500/) and their founding years:
1. Walmart (1945)
2. Berkshire Hathaway (1889)
3. Apple (1977)
4. Exxon Mobil (1870)
5. McKesson (1883)
6. UnitedHealth Group (1974)
7. CVS Health (1964)
8. General Motors (1908)
9. AT&T (1983 as AT&T, 1877 as Bell Telephone)
10. Ford Motor (1903)
By market cap (2017, first quarter) (https://en.wikipedia.org/wiki/List_of_public_corporations_by...)
1. Apple (1977)
2. Alphabet (1988)
3. Microsoft (1975)
4. Amazon.com (1994)
5. Berkshire Hathaway (1889)
6. Exxon Mobil (1870)
7. Johnson & Johnson (1886)
8. Facebook (2004)
9. JPMorgan Chase (1871 as J.P. Morgan & Co)
10. Wells Fargo (1852)
I'm not listing these as counterfactual, but only as an example. I know there are different ways of measuring "top" companies, and these are two. It's also arguable that these don't capture the idea of "top innovators". I'm interested in knowing what you're using. I'm probably missing something obvious.
https://en.wikipedia.org/wiki/Berkshire_Hathaway#History
Notably absent from the list are behemoths of the past, like RCA, Kodak, Sears, IBM, Ford, GM, US Steel, Union Pacific, etc.
If they need something done and they can't guarantee long-term employment, they can buy commercial services for what they need. Engineering firms and temp offices will be happy to send you a secretary, a software solution or a CAD file with what you asked.
It's really not your business what other people freely choose to do with their time. Nobody makes anyone work at a startup.
I personally love working on new, potent ideas, but this is not a viable model of development for society. "Gig economy" is getting out of control, and it's better to kill the startups than to let people abuse the legal loopholes they require.
If you cannot provide a certain minimum of employment time, you shouldn't hire people. As for short-term contracts, they were misused a lot post-2008, but the situation is much better now and most people won't work under such contracts.