We don’t have to. Doesn’t mean we won’t as part of a free trade agreement. Will we be able to make a free trade agreement with India without relaxing control of our border with India beyond what it is currently as an EU member state? Who knows, negotiations haven’t taken place yet but that is certainly what India wants. What happens if India won’t budge on this and we have to revert back to WTO rules? The EU will be on stronger trading terms with India than the UK and that could seriously damage the export market making our goods more expensive in India than EU goods and also mean that duty will be paid on all goods coming from India increasing prices to business and the consumer. It won’t just be the EU either, they also have a GSP arrangement with the USA for instance, not sure if that’s free trade or heavily reduced duty but the points the same.
Basically everyone knows that if we don't leave on amicable terms with the EU we're up Shit Creek without a paddle and they will use the situation to their advantage.