FTO's use uniforms because it impresses the customer, the prospective student. When far too many students in FTO's are only there because they think they will look cool in a jet cockpit, what better way to help them part with the cash if the school can let them see that they will at least look like a pilot on day 1.
As for stripes on students,
Again, same reason, it appeals to the vanity of the student if they can get to wear some bars. Never mind that they don't have a job, they will feel like real pilots with 1 bar per 100 hrs.
It's all marketing, all to do with encouraging people to hand over money. And given that some FTO's are inhabited by people like
dhps and the people alluded to by
Finals19 , it is an important and effective piece of marketing.