It's not just that existing companies resist...
The companies there were starting up in the early 20th century had to pay very little in the way of taxes, unlike companies starting out today whose first 1/3 of profit goes for income tax. Similarly for regulations -- there were virtually none a hundred years ago. Thus older companies were able to grow large without those burdens and new companies today face not only those burdens but entrenched competitors as well -- they have to beat out the best of the best, not just others that are just starting out.