I see what you mean, but in that regard they never mattered (i.e. nothing changed in 2020). Parties could always pressure candidates to drop out of the race before it was decided.

In 2020 there was a lot of pressure for the Dems to pick a candidate early to beat Trump. It was a strategic decision to solidify the base and give the impression of party unity. The bickering between Hilary and Bernie camps in 2016 was thought to be a major reason Trump won so they wanted to avoid that.