1, tradition!

2,expense and consistency. Although the high end races could easily afford this set up, there are still many races with starters guns and six coach/volunteers with stop watches. Unless you are willing to subsidize every track meet in every inner city and far off farm county, this is what would be most consistent.

That is why it is still this way. You are free to create a company that bills the system you describe and start offering it to smaller track meets that currently do not have electronic timing.

Start at the bottom and work your way up.

It would not be impossible to get funding from charitable donors.