Because love is the equivalent to zero. The origin of why this term is used has never really been proven. However, some theories include a mispronunciation of the french word l'oeuf (meaning egg, or goose egg, which is slang for zero), the idea that players play for the "love" of the game, or that the upper class players used the term to confuse lower class individuals.
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This is directly from the website tennis.lifetips:
"In tennis scoring love means nothing, hence love-thirty is 0-30. A common (but unproven) explanation for the term "love" to signify a score of zero is that it originates from the French term "l'oeuf." Another explanation is based on the idea that to do something for love is to do something for nothing (zero)."
The origin of the use of "love" for zero is also disputed. It is possible that it derives from the French expression for "the egg" because an egg looks like the number zero.