More to the point, why would 37signals ever
want to take investor money, much less
need it? Surely, the founders are wealthy enough now that getting more would just be bouncing the rubble (see Jason Cohen's "Rich vs King"), so money is unlikely to motivate them. And from reading Jason Fried's extensive writings, it's very clear that they're more interested in making a company that's a truly excellent place/way to work than they are with making a company big.
The only reason to take investor money is to grow the business faster than you can with cash flow alone - or to sell it in order to become wealthy and/or go on to the next big idea. The 37signals apparently don't want either of those things.
I can imagine a line of venture capitalists basically begging for them to take some investment, and them just refusing.