Why Cricket ball is so heavy??

Philip Hughes it was.And an Umpire was killed in Wales a while back.

It isn’t really what you mean but it is what the ball is made from...
https://racketrampage.com/why-are-cricket-balls-so-heavy-reasons-revealed/

And would it be the same game if softer balls were used?
No...although tennis ball cricket is popular in India.
I watch cricket and I dread losing sight of the ball if it is heading into the crowd.

I suppose asking why a hockey puck is frozen is out of the question.