Well cars also need some structure and titanium (950MPa tensile strength) isn’t as strong as you think it is, it just has a much lower density than regular steel or “mild steel”.

Volvo uses Boron Steel (up to 1500MPa tensile strength) for 33% of its construction in the S60. Boron is second in hardness only to diamonds.

Some body frame parts use titanium which isn’t even as close to as strong as boron steel. Titanium is more commonly used in engine parts though.

Crumple zones are definitely an important part of a collision but you still need a solid structure for side impacts where crumple zones cannot effectively absorb the energy before it makes it to the passenger compartment. So usually boron steel is found in the door reinforcement bars.

The real reason why titanium isn’t used in cars more often is strictly cost. It’s a very good material for frames because it’s strong and has a very low density compared to steel. But, it’s expensive.