So that he can become stronger.

Goku is at the low-end of the top 25 strongest beings in Dragon Ball Super.

There are 18 Angels above him.

Beerus and maybe one or two other God of Destruction above him.

Frieza

Maybe Gohan

He and Vegeta are on par with each other.

So as you can see he has a long way to go to be the number 1 strongest. And the journey wouldn’t be fun if Goku were already number 1. He needs to grow constantly and break his limitations. It makes the series more fun and exciting and most of all motivating for us in the real world. As Beerus said, there’s always gonna be someone stronger than you. Roshi said this in OG Dragon Ball too.

The positive way to frame this is to look at what Goku was previously and compare that to the present. If we were to compare Super Goku to Namek Saga Goku, you can imagine the difference between the levels of power between the two. Goku has grown to be on the level of the Gods, his strength is beyond anything he was previously. That is growth. Goku may always be weak in the present but he’s a God to the past and the future is right within his grasp because he won’t stop growing. Even he himself is excited he’s at the bottom.

And remember, Goku achieved something not even Gods themselves did.

Goku is extremely weak just so he can become extremely strong and on and on it goes.

Good day