It's naive to think that a company that has been operating for decades only has capability that is implementable in a weekend. Surely there are plenty of edge cases with certain songs/genres and how they mix with various environmental noises. That in addition to already having the system scaled, and having a team of people who are intimately familiar with maintaining that system.

Then there's also the userbase. Even if you successfully clone the app, most Shazam users won't even know it exists.