Why do some people say not to trade contracts?

Because contracts can help you quickly identify your trading issues. In short: those who advise you against trading contracts are concerned that you may not manage your position well (or they themselves may not manage it well).

If you have any of the following shortcomings in your trading: making random judgments and frequently operating, holding onto losing positions, increasing your position against the trend, not being able to calculate the liquidation price clearly... then contracts will quickly tell you that you are wrong, rather than like in spot trading: "You just had bad luck in this cycle, but you will definitely achieve financial freedom in the next four-year bull market cycle!".

If you trade ten times a year, then your sample size for the year is only 10, making it difficult to summarize any patterns;

If you trade ten times a day, then your sample size for the month is equivalent to the previous 30 years, providing many more opportunities to help you realize your trading issues, not to mention that for short-term trading, trading ten times a day is relatively infrequent.

Those who believe that trading cryptocurrencies will ultimately lead to liquidation essentially do not understand the math.

Someone who can't even calculate the liquidation price, saying that you will be liquidated, is like you eating dumplings dipped in soy sauce; it tells you that a person drinking a bottle of soy sauce will die from lack of oxygen—don’t you think it’s salty (idle)?