Last I checked, these phones are full of glass. And glass is incredibly rigid - far more rigid than steel (and lighter than steel too). Also, glass doesn't block light or radio waves or magnetic fields... which is the why basically all of modern phones are full of glass.

Anyway, since there's so much glass, the titanium doesn't need to be rigid. It just needs to be able to handle an impact reasonably well and look nice. Titanium does both better than steel.

The primary metal in these phones is Aluminium by the way. That metal is very soft and doesn't look great especially once you've dropped the phone. There's a thin Titanium outer coating somehow bonded to the Aluminium - they have a single piece of metal made of with two completely different metals. Clearly Apple isn't afraid of making a phone that's difficult to manufacture.

There's probably 20x more aluminium than titanium in the phone. Aluminium is very light... and it's drawback (scratches/dents easily) is eliminated by the 1mm of titanium bonded to it.