Why does the tofu keep turning out brittle

Boiling tofu causes the bean curd to exude moisture, so you are correct there is a structural difference between boiled and unboiled tofu. The heat tightens up the protein structure, hence affecting consistency. Boiling bean curd for braised and fried dishes is a long practised technique in Asia, partly for these reasons. Because of the structural change, boiled tofu can stand up better to more vigorous cooking methods like long simmering and frying.

The type of tofu (mainly its coagulating agents) are related to the efficacy of this method, but it absolutely applies to your typical firm supermarket brands (non-silken).

My girlfriend and I cook a lot of tofu - we have also found that "firm" and "extra-firm" tofu is highly variable and that the quality makes a big difference in the actual firmness, density and cooking results.

Here in SF though we did have good luck with the random brand of tofu available at our local produce market (in the Outer Sunset) we have now switched to almost exclusively cooking with tofu from http://www.tofuyu.com/ who are a local tofu maker and whose plain tofu cooks up amazingly well - gets great color, stays firm and in short is far better than anything else we've tried.

While not every part of the country is as lucky as the Bay Area to have multiple artisanal tofu makers competing to make fantastic tofu you should experiment with the various brands of tofu you can find - look for the one that gets the best results for your in your preparations.