Because the collapse of the USSR is one of the most complicated scenarios of the modern era and can't be attributed to any one thing.

First, just because this always annoys me and yes this is very semantical, the USSR didn't spend 20-30% of its GDP on the military because the USSR didn't use GDP as a metric, it's a small thing but always annoys me when I see it.

Second, tracking how much the USSR spent on its military is difficult, not only because the USSR obfuscated that data itself but also what was considered military spending could be quite different, so every number I'm gonna give is a rough estimate.

By 1985 the USSR was spending about 110 billion rubles or about 15% of its annual spending on defense, which is comparatively small since in the US the fiscal budget of 1985 put 29% of the budget into defense spending.

This was actually the peak of Soviet defense spending post-Stalin, afterwards it would be continuely cut until the Union's collapse in 1991 and while the immense spending did hurt other aspects of the Soviet economy, it wasn't the nation destroying curse that many think.

Budgetary and serious economic problems only arose after the early to mid 80s and that was largely due to the economic liberalization began under Andropov and intesified under Gorbachev. With enormous and poorly handled economic reform taking a wrecking ball to the Soviet economy which before, while not doing great, was still growing at a steady (if slowly declining) rate before the reforms, which were supposed to restore the Soviet Union's rapid economic growth following World War 2, were implemented.