They never stuck to the border agreements because for starters the surrounding arab countries (can't say Palestine because it didn't even exist as a country back then) refuse to acknowledge the borders and instead wanted to eradicate Israel. And for the first 30 years of after Israel was officially founded those same countries kept poking and prodding hoping to force them out in a similar fashion current day Israel does to Palestine. Is it really a wonder then that they treat Palestinians like dirt and ignore border agreements? It's literally how they were historically treated.

And this isn't to defend Israel. Current day Israel is the reason Hamas exists and Israeli treatment of Palestinians (not just in Gaza) laid the foundation of the terror attack. What I'm pointing out is that there's a clear circle of violence that has been perpetuated by generations on both side and this nuanceless "Palestinians / Hamas is evil" or "Israelis are scum" perspectives are perpetuating the same violence by dehumanizing the other side.