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.
Agreed, I believe
smart trader is what he really want to define and not investors. Investors always enter to a business that has a future price growth regardless what is the current as long as it show a good sign of future gains.
Answering OP, The reason why people still entering while price is high is because of the word "
Opportunity". Most the traders wait for this price pump for so many years and do you think you can wait again another 3 years to buy cheap BTC while you know that BTC can go higher on the current price due to its previous ATH?
And also, in trading, The best way to enter in to a position is when there is a clear price trend. On this case, BTC is on bull run and riding the wave is the safest way of trading rather wait for low price to buy without any confirmation.