Right now it's entirely timestamp-based. That means it can interface and work on simple playback terms. On time, jump, jump to time, etc. Having to get frame data and hash it, and make playback depend on it adds a lot of technical complexity.
If ad length varies you don't even know how far to jump ahead. And if you haven't prebuffered the data until after the ad, you can't find out from a hashed after-frame-hash-value either.