Study Gregor's reaction after the metamorphosis, and you will understand.
Of course, from most people's point of view, Gregor's reaction is a bit weird. But the situation in which he suddenly finds himself is also very weird. It's quite possible that it's normal to behave weirdly in a weird situation.
Besides, what could he have done? Use Morse code to give interviews to newspapers? Perform in a circus?
It should also be noted that Gregor was already behaving weirdly even before his transformation. He practically sacrificed himself for his family and lived only for others, not thinking of himself, his wishes and desires. Helping your parents is fine, but in his case it was too much.
Looking at his abusive father, one might assume that little Gregor's parents only accepted their son when he behaved as they wanted him to. They didn't really love him. If they had, after he turned into a giant insect, his parents and sister would have felt sorry for him and worked with Gregor to see how they could turn the situation to their advantage. With a giant bug in the family, they could have a career in both science and show business. But these people with small minds lacked the imagination for that.
I think Gregor was made to realize from an early age that he was only worth anything if he was useful. When he turned into an insect and could no longer work, he felt like a burden to society and his family. He couldn't be useful anymore, so he didn't care about himself. That's why he behaved so strangely. Also, as a giant insect, he became a great shame to the family and had to be hidden away. He would probably have been hidden just as carefully if he had been maimed in an accident or become mentally ill. Anything that doesn't fit into his parents' primitive world view has to be hidden and then eliminated. So Gregor was wounded by his father, ignored by his mother, and even his sister wanted him dead. So he finally died -- and everyone was relieved. So it was Gregor's parents, not Gregor himself, who were cruel and weird.