I think there are many reasons why people still buy bitcoin. one may be because people are more likely to increase the value of bitcoin so once they invest in it they can easily grow their investment and think that it is easier for them to live up to their lifestyle
Only a fraction of all the bitcoins that have been issued today are sold in the exchange market. The Bitcoin market is competitive, meaning bitcoin prices will rise or fall depending on demand and supply. Moreover, the new bitcoin will continue to be published in the coming decades. Therefore even the most ambitious buyers will not be able to buy all the bitcoins that are present today. However, this situation does not indicate that the market is not vulnerable to price manipulation; still does not require a lot of money to move the market price up or down, so so far Bitcoin is still classified as a volatile asset.