There are two points to consider.

First, the God/Lord distinction is very important. 'God' is not really a proper name here; it is theos, which means 'divinity' in a general sense. Given its abstract nature, it is not easy to always define what this means, but 'Lord' on the other hand clearly has to do with the authority to command and determine the destiny of subjects. Theos is broader and would include things like one's object of worship. A deity can be your 'Lord' (take authority over you, by force) without being your theos/God. So the distinction is important.

A more important factor is that Jesus is arguing against his opponents, the Sadducees, who believe that there is no resurrection, and no afterlife, by saying that a non-existent person cannot have a theos. He actually is only using 'dead' because he happens to refer to dead people -- Abraham, Isaac, and Jacob. But again, his real point -- in light of what his opponents claim -- is not so much that these people are dead (or not), but that they are not non-existent in death. So though it is the same Greek word for 'dead' in both verses (nekros), it carries a different semantic range in Matthew than it does in Paul, who is asserting Jesus' Lordship over those who are living and dead, but in the latter case, not non-existent.