Because Olmo is made of glass. Of course he would start if wasn't so injury prone.
Indeed a top, top player. Injuries really are a disease to football. Olmo is next in the line of exceptional footballers whose magic we get less of because he lacks fitness.
He should be treated like your best set of dishes that only come out when some important guests visit your house. For the rest of the time, keep it in a box, with foam padding.
There must be a tailor made solution to his injuries just like there seemingly was for Pedri and Dembele.
We could treat him like glass and limit his playing time but I think sooner or later he'd break again unless something different is being done behind the scenes.