Back in ye olden days the federal government as written/created did not take into account 50 states nor a 2-party dominance in the political spectrum; it hasn't changed to accommodate society over the years.
You see this idea fairly often, and it's a wrong one.
The Constitution can be amended. It's been amended 27 times, and amendments can be amended by other amendments.
Beyond that, it's historically innaccurate that the crafters of the constitution or the founders of the United States were incapable of considering a "2 party system" or the idea that the US would have 50 states. Since about as early as the nations founding there was a desire for the country to grow as it became larger and more powerful. And, since even before the founding of the US, the concept of fairly static political parties was ingrained in the DNA of the nation. Remember, the US is a bicameral legislature and is a descendant of bicameral legislatures; the concept of two representative bodies is a normal concept, and "parties" extending from that is normal. And the concept of two distinct groups was as common when the Constitution was being crafted as it is today, between Republicans and Democrats, as most prominent politicians belong to either the Federalist group of Anti-Federalist group. This was the defining division of the country for the first 40 or so years of it existing, before proper political parties took over.
I think it's also overstated how much the parties speak with one voice. As fucked up as the Republican party is, right now, remember, there were
18 candidates for president under the Republican party in 2016... And at least 6 (or more) who were
very distinct from one another. Ultimately, that's how a goon like Trump became president, by being a strong-arm in a field of competitors.
Beyond that, though, the founders of the country and the framers of the Constitution weren't idiots.