Chiefly because if you have a fast machine, good connection, and a nearby CDN node, ~10 ms may be enough to establish a connection, make a request,and see the response.

Plus, if you do setInterval(..., 1) and then measure new Date().getTime() deltas, you will probably see that even if the browser isn't doing anything taxing, you get 100 ms or more every now and then...

I'm assuming there is someone working on JavaScript-RT as we speak, though ;-)