They’re much stronger than you think and even a 9-pound catfish really tests your endurance and strength, moreso with every extra pound.
You also have to be sure your line doesn’t snap or the fish doesn’t unhook, whether you should be reeling or letting it run, tons of variables.
And there’s so many types of fish and they all act differently, have different diets and preferences, every bait and fly works amazing on one fish and is terrible for others.
For me, the battle with the fish is probably the most fun, and if you win, you get to eat hotpot or roast fish or whatever.