Do you know why/how?
The only projects I can actually think of that are based in the US and made by american authors are those by xiph.org, and they only get away with it because their entire business model is developing royalty free alternatives to MPEG codecs.
Even if multimedia patents might not affect big corporations much, they definitely seem to strongly affect the open source community. I imagine if we had similar dystopian laws here in the EU, our best and most beloved multimedia software would plain not exist.
So a good portion of the startups you're imagining die early due to a lack of investment or incorporate in the US at some point in order to have a better chance of finding investors. And then some of the rest simply aren't as growth-focused because they need to focus on short-term profitability to survive, which means they'll likely end up silently dominating a particular industry niche rather than making a big entrance on the global stage.
Software patents are only a problem if you can survive long enough to be sued. If you're not exceptionally unlucky, you're more likely to go bankrupt before that happens.
There's an interesting loophole: just also include in your invention the machine (memory + CPU) running the algorithm. Then you're patenting a device, instead of simply math.
Stallman described this method more than 10 years ago. I was hoping this wasn't true, until the (french) company I work in successfully used it to patent an underlying algorithm from the software I had written.
Maybe such a patent would be debunked in an european court? I'd be glad if this were the case.