Being a smart investor doesn't necessarily mean that you only buy when prices are "cheap". You can still be a smart investor if you invest in businesses or assets that you think will be worth a lot more in the future(assuming you've done the research), regardless of current price.
To answer your question, it's pretty simple. People buy at current prices because they think it will be worth more in the long term.
Also, define "high"? Bitcoin being at 15k, which is approximately 5k away from the all-time high, doesn't make it's price "high". Personally, as a dude who's long-term bullish on bitcoin, I'd even say that 20k is "cheap" when talking about potential future prices.
But it really depends on the intended profit and how long one's willing to wait to get that profit. As for me, the price is pretty high right now, and I am not a person who dreams of Bitcoin worth $50k or more. I prefer stability to bulls and bears. $15k is a very good price, and I'd be glad if Bitcoin just stabilized here, the fees normalized and more places started accepting Bitcoin as payment. I would like to be able to use Bitcoin without selling it for fiat all the time. And if the price has to be lower for this, I don't mind. Bitcoin is still largely considered an investment, but I think it undermines what Bitcoin is.