Because most of this infrastructure was built in the 50s and 60s when the government actually had money to spend due to a 75% corporate/millionaire tax rate and invested it in infrastructure and improving life for Americans. It's impossible to make this infrastructure indestructible and it was always meant to be maintained or failures are unavoidable.

Unfortunately Republicans (esp Reagan) gutted the government's income (by slashing corporate and the obscenely rich's tax rate to near zero), deregulated everything, ended infrastructure maintenance as "wasteful government spending" and basically sabotaged the future of our country. Things will only get worse as infrastructure is more and more stressed and out of date/neglected and the climate changes.

These accidents are becoming more and more common and that trend will continue, and as other countries keep pace with modernization the US will fall comparatively further and further behind until in many ways we're no longer a first world nation, at least in comparison to all others. It's inevitable with the path we're on, which shows no signs of or even avenues for changing. In fact in almost every single respect we're already dead last on every single metric of first world nations.