"Why do we sit back and let this happen to us?"

Answer: we don't really know what to do. Almost any solution we can think of requires the right politians in power, quite a while to work its magic (for better or for worse) and/or a large number of participants willing to literally lose everything overnight.

Immigration moratorium? Requires the right politicians in power and the political will to enforce it and keep it.

Deport all the "problem" demographics? Requires a large number of people willing to lose everything and be called names and a plan for locating and transporting people as well as where to send them. Likely requires help from willing government bureaucrats, too.

Go down to the border and create the sorts of self-defense scenarios that would allow us to literally take the invaders out? Be ready to lose all your life savings and/or (a few decades of) your life "defending yourself". That, too, likely requires help from a sympathetic government.

Pay native citizens to marry and have children? We already do that. It's called tax breaks and the government welfare system. The only real improvements we could make to that is extra welfare money for intact biological families, banning direct descendants of non-citizens (AKA anchor babies) in addition to non-citizens from receiving benefits and finding ways to flatten the "welfare cliff". There are probably charities that help, too. We need to find or make a list and spread it around. Anyone around to provide extra funding to these groups?