re: With today’s tech, can’t we just cut right through

Go to Google Maps, ask for Panama Canal, then hit the "Terrain" option to the right of the satellite icon in lower left.

Setting aside environmental considerations (critters are doubtless hitching rides on the hulls of the ships), there are massive mountains pretty much everywhere. Those mountains aren't stable and you take many tons off, and some faults may start sliding.

Locals may not want to be displaced, mountains in national parks are not going to be deforested without environmentalists' blessings. Illegal aliens would even look fondly back on the days when the Darien Pass was the problem with their trek from Colombia.