Why TQQQ is not good for buy and hold type long term investment? I understand it can be more volatile as it can go up and down 3x of QQQ but its chart shows 17,000% return over the past 10 years.

Just sharing from own experiment on leveraged ETFs.
Bought and held the tickers.
TQQQ is bull.
QID is bear.

Feb 2020
TQQQ and QID roughly same dollar amount.

Jan 2022
TQQQ +$300
QID -$30

Results will vary as the situation changes.
And there are stock splits in between.

Not simple to just buy and hold if you don’t understand how they work.

Others have explained well. I hold TQQQ for 1 yr+ (buying after march low) so return is ecstatic that i have to buy other unleveraged stuff to rebalance.

But it’s true that when market is sideways, you lose money (oct 2021 - now). And you can search in google for a hypothetical backtest of TQQQ up to dot com bubble, if you invested in hypothetical TQQQ at its ATH before dot com bubble, you may have 99.9% drawdown, and you haven’t reached BEP even until now (even with those stellar returns at the longest bull market in history). We may not have the balls to see $100 becoming 10 cents.

I suggest you combine it technical (simple moving average or anything) to give you time to buy / sell. Lets say auto sell and switch to Bond etf when it cross down the 200 SMA (just an idea) - certainly not financial advice and only an idea, i believe they never cross down in the last 1+ year since I held it.