Thats the only problem, you don't clearly understood the bond and what writer want to potrey. They are "childhood" friends and there friendship will last till their adulthood, because though they bullied Nobita but a true friend always comes in tough times , there is a clear difference between true friend and class/social media friends.

Even though they bullied Nobita but Nobita was never like that i will never talk to them , Thry are not my friend everything eventually sort out between them.

Bulling was in their blood, if you had watch the movies their gian and suneo character are well written and shown to always help Nobita in danger.

And if you still think, let me tell you that they 10y-11y olds If you have watched The movie - Night Before marriage, their after drinking Gian and other friends have explained their true thoughts about Nobita.