Whilst it may seem that the EU makes it difficult for member states to trade with non-EU countries, this is completely incorrect.
Being a member of the EU does not restrict trade outside the EU. In fact, being a member of the EU makes it easier to trade with many other countries because of the EU's free trade deals with other nations.
Whilst it may be much easier to trade with the EU member states than it is to trade with the rest of the world, that is because the EU has broken down the internal barriers between members, not because it has put up external barriers. This is why the UK, and other EU member states, manage to trade very well with non-EU states.
By leaving the EU, the UK would be putting up the same barriers to trade with the EU member states that it faces when trading with non-EU states. And if the UK leaves with a bad deal, or no deal, it will be putting up even more barriers to trade than it has ever had before.