It's not just Uber of course. There is tons of money flooding into questionable tech startups with no end in sight. In 2014 bitcoin companies alone had received more investment money than the whole internet did in 1996. It's an industry wide phenomenon.
The total amount of VC money invested in the US in 2015 was $58.8b [1]. That is less than the operating income of Apple. It is a lot of money in some respects, but the potential rewards are also huge (look at Apple!). I think the huge valuations have more to do with the huge potential for global businesses than whatever Yellen is doing with interest rates.
The internet was a completely different place in 1996, I don't think any fair comparison can be made regarding investment. Something like a billion daily active users and the cash that can generate wasn't a possibility. The whole internet advertising business in 1996 was estimated at $267m [2] which is less than Google now brings in PER DAY (~$283m).
[1] http://nvca.org/pressreleases/58-8-billion-in-venture-capita...
[2] https://www.iab.com/news/internet-advertising-bureau-announc...
The answer is always going to be some variation of "the goshdurned gubmint". Maybe interest rates, maybe someone read Greenspan's tea leaves which he deliberately left sitting out to cause "malinvestment", maybe something else entirely.
How do you explain things like the .com boom when interest rates were "normal"? Uber has raised a lot of money, but it's a drop in the bucket in the world of global finance (and a very small drop at that!).