Spirit is still there, but it takes more money to make money [start your own biz]...and more money to live...than it did then. After all, a cook in a diner used to be able to support a family. Now it takes both parents and the kids and the dog working to maintain the same standard of living [modest home, etc] as it did in the 40's,50's/early 60's.
Not only that, but the independent business [grocers and many small restaurant/diners for example] have been driven out of business by the mega corps/chains.
Corporate America knows what it's doing. It devours anything in its path. So it's not quite as simple as "what happened to the entrepreneurial spirit of the past."