Why would you only have one copy of a card in Magic the Gathering?

In MTG, when building your deck, what is the logic of having only one or two copies of a card in your deck. Why not have 3 or 4. It seems to me that if the card is good enough to earn a spot in your deck, then you should ensure that you see that card during a match which would suggest 3-4 copies of it in your deck. I understand having only 1 if its a legend because of the legend rule. But why would you only have 2 copies of a card versus having 4 so you will maximize your chances of getting it. I seem to have this trouble when building my decks in that I always have 1 or 4 copies, never in between and the cards that I only have 1 copy of, are always legends.