By this line of logic it would be a mistake to buy Microsoft shares on day 2 of trading, Apple shares on day 2, 'insert any successful company' shares when they

By this line of logic it would be a mistake to buy Microsoft shares on day 2 of trading, Apple shares on day 2, 'insert any successful company' shares when they're higher. People buy shares because they think it will be worth more in the future, they think the company will be worth more than it currently is. People buy Google at it's current market cap because they think it's going to be worth more.