because adam smith who coined the term and theory himself said it was flawed and simply wasn't correct, marx chose to ignore it and insisted that it was correct because "it just gotta be right bro, just trust me otherwise all my other theories fail." ltv fails because: doing labor that's not valuable and not desirable, doesn't create or add value, therefore, labor is not equal, it must be desired in order to have value. however, even if it's extremely desirable, as in, necessary for life, like water, it can still be extremely cheap if borderline free to the end consumer, or if it's worthless beyond "i want" and personal taste, it can still fetch a very, very high price. adam smith himself used this example to go "this theory is invalid simply because of this truth, however i have yet to figure out a better explanation."  the subjective theory of value is a complete theory that completely understands and encapsulates why people value things" they're willing to part with their time, effort, and money in order to acquire it. that's why two cardboard cards, identical in all ways except the ink pattern, can be worth a penny, or 500 dollars. no greater or lesser labor between the two, merely personal desirability. food, water, all pretty cheap, despite taking a fair amount of labor, while you claim most people can just grow their own food, i doubt most modern urban people would survive to grow enough food for themselves. it's very hard work (communists wouldn't know though since they think you're suppose to kill farmers, take all their crops, and don't understand why it doesn't come back next year) labor is not a tangibly desirable thing, you can do the (effective) same labor many times and achieve extremely different outcomes based on subjective values, thus, labor does not determine value (it's a mere part of the equation), it is instead, the subjective value people personally have.
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