This is to elect the President. In a presidential system, as in the US, you choose the leader of the executive portion of government separately from the legislative leader. In a parliamentary system, as many countries in Europe use, the public doesn’t choose the leader of the executive portion of government. Instead, they just vote for representatives in the legislative portion, and then those legislators form a coalition (if necessary) and choose a leader of the executive (the prime minister). The closest analog to coalition forming in the presidential election is doing exactly what the Greens are proposing above – having a candidate drop out and endorse another, with the hopes that they can sway their supporters. It’s basically what JFK Jr did, for example, with Trump.

While hypothetically the US could form legislative coalitions, in practice, due to the way the US electoral system works, US parties are essentially equivalent to electoral coalitions in parliamentary systems already – we already form “big tent” parties necessary to control a house. In the US, the closest analog to this sort of thing actually happening after the elections is when you hear about something like “an independent legislator who caucuses with the Democrats”. The US also has weak party discipline compared to many countries in Europe, so legislators are much less constrained to vote along party lines anyway.

Different systems, function kinda differently.