Because a tablet--whether iPad, Android or Surface--can do something no full laptop can do; they can be used while walking.
Yes, I do know that there are "convertible" laptops out there and for the most part they were very niche tools in very niche markets which until Windows 8 had effectively no available software that took advantage of that 'walkabout' capability. Even then they tended to weigh five pounds or more and cost in excess of $3K where both the Surface and the iPad start at only $500 or 1/6th the price
So no, a laptop cannot "do everything both can do." If you just like carrying five pounds around under your arm to proclaim, "I'm better than all you fanboys", that's certainly your prerogative, but almost everybody else will just shake their heads when they see you.
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