You shouldn't say this to people like it's some obvious truth. There are many cases in which this action will land you in trouble due to it not being a legally valid termination of the contract (which of course may be different by country -- it's very common that cancelling requires an actual message to the other party).

One specific example is if your contract has a termination period, which is pretty common, at least in my part of Europe. If you simply stop paying, you are denying the other party N months of revenue (your cancellation period) that you are contractually obliged to pay. You are now defaulting on your payments and will likely pay additional fees.