Few people can build good DIY scopes. Just think about delay times and phase shifts for a 100MHz scope. And think about the cost of a gigasample/s-kapable ADC. Of what it means to implement a staggered read where you have a fast sample-and-hold circuit but a slow ADC and capture repetitive signals by capturing the same curve multiple times with a time-shift between each pass. Think about the problems just 10ns of extra delay in your trig circuit will introduce.

It is very easy to do low-end scopes - basically reasonably fast data loggers with a low number of bits of resolution. But if you step up the analog resolution or the sampling frequencies, life isn't so fun anymore.