If the product works and you start to take market share from the incumbents, then they will seek you out. Market share is your leverage.
If you go straight to them with a product that isn't a huge step forward, you'll either get ignored, get a lowball offer, or get squeezed you out as the competitor injects money into the app.
In general, I don't think it's a skeevy practice to form a company expecting to get acquired.