It's because they would easily warp from heat stress or bend easily even when cold. Titanium on the other hand is as light as aluminum and 3 times as resilient as steel. This is why Akrapovic exhaust are made from titanium, a metal that also doesn't rust or easily oxidize.

Exhausts. Akrapovic Titanium Silencer Complete 4-1 System - Road Legal.

This has led to performance gains in the mid and high rev ranges, with an increase in power of 0.7 kW (0.9 hp) and a torque enhancement of 1.3 Nm – both at 5,700 rpm – when compared to a standard stock system and tested on the Akrapovič in-house dyno.

Akrapovič d.d. (Slovene pronunciation: [aˈkrapɔʋitʃ]) is a Slovenian manufacturer of exhaust systems that began in 1990 in the motorcycle market, and expanded into automobile exhausts in 2010.