The advantages of staying in are, amongst others:-
Retaining simplified arrangements for the movement of good across borders - no transit documents, vehicle permits, customs clearance, outlaying of VAT up front on import, exports clearance.
Free movement of people allowing Brits to live and work in other EU States.
Freedom of UK based airlines, especially EasyJet to use UK registered aircraft to operate internal EU services - setting up a separate company within the EU would negate the problem, but would be an increased cost to their businesses.
The laws relating to maximum working hours, parental leave, TUPE - all of which a Conservative government would undoubtedly repeal within hours of being released from the EU.
Freedom to buy goods across borders when travelling without incurring the wrath of HMRC (and bills for VAT and excise duty).
...................if it is so good, why do we never hear any of this from the remain campaign?