Because that's the rules..
Article 50 – the only established legal way to leave the EU. Once triggered, there is no turning back, and it will exclude the UK from key decisions as well as the final vote. It leaves the EU in charge of the timetable during two years of negotiations, following which the UK could easily be presented with a ‘take it or leave it’ deal. UK lawyers helped draw up the agreement.
As for punishing the UK, that's simply a backwards way of looking at things. If Brexit opens up opportunities for others in the EU, as a member of the EU, paying their way, they will have every right to veto any deals if they aren't in their country's best interests. Trade between EU members will always come 1st. It's not about getting one over on the Brits, or tactics, or anything underhand, it's about being part of a Union. Its what we've done so well out of since joining.