This question would have an answer for every individual if it weren't so traceable to a single cause: because people just don't teach that.

There's no reason to think that historians know less about that period, and we're not going to get into the whole question of quantifying how much we "know" about things in the past.

Let's look at the time ranges some High School standards cover

AP World History (Modern)	1200 to present
AP European History	1450 to present
World History (MI)	300-1500 in one unit, 1500 - present in remaining three
World History (TN)	1750 to present
Modern World History (OH)	1600 to present
World History (CA)	1750 to present
Global History and Geography (NY)	Grade 9: Paleolithic to 1750, Grade 10: 1750 to present
World History (FL)	Medieval and non-European civs: 41 benchmarks, 1600-present: 71 benchmarks
World History (TX)	8000 BC -1450 AD: 3 standards, 1450 - present: 10 standards
IB History	12 topics, just cover 750-1500

Now, I've only included high school standards because honestly, who remembers anything from middle school (let alone high school...). Many of the states listed here teach some amount of earlier history in Grade 7, with a unit on the early USA in Grade 8.

Tennessee does include an Ancient History elective; despite having standards for many other social studies electives, the rest do not. That Ohio and the AP program call their classes "Modern" is cute, but misleading since it's the only one.

That's not to say that standards that do address earlier eras are great. I've discussed here, the issues with the way non-European groups are given static cultural descriptions while Europeans get narratives focused on change and development. For instance, we get this when talking about "Classic Civilizations:"

Students will investigate how geographic factors encouraged or hindered expansion and interactions within the Greek, Roman, and Mayan civilizations.

but when it to the... "not Classical" ones:

Students will examine the adaptations made to the environment by the Aztecs and Incas.

Nevermind that both the Aztec and Inca states were more expansionist than the Classic Maya or Greeks, the language of "adaptation to the environment" makes these great American civs out to primitive.

Often when pre-1500 content is included, it is greatly compressed. There are nearly twice as many content benchmarks in Florida standards for all Unit 3 (Medieval Asia, Medival Europe, Byzantine Empire) and Unit 2 (Islamic, Mesoamerican, South American, and African civs) as there are for the Rennaissance and after. Heck, Unit 6 (Western nationalism, industrialization, and imperialism) has just as many benchmarks as Unit 3, and more than Unit 2. Though New York is unique in its Grade 9 standards, there is just as much content included for 1400-1750 as there is for everything before. And what there is of those is not great; some seem like they were pulled from a manual for Civilization V:

A period of peace, prosperity, and cultural achievements can be designated as a Golden Age. Students will examine the achievements of Greece, Gupta, Han Dynasty, Maya, and Rome to determine if the civilizations experienced a Golden Age.

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Why is this? Well one can be the cynic and note the nationalist, Western agenda in many state standards. Consider that the Texas economics standards literally begin with:

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Economics with Emphasis on the Free Enterprise System and Its Benefits is the culmination of the economic content and concepts studied from Kindergarten through required secondary courses.

and that this standard:

The student understands the development and use of radical Islamic terrorism in the second half of the 20th century and the early 21st century.

has the same priority as this:

The student understands the causes and impact of World War II.

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But that doesn't quite solve things. Extrapolating Eurocentricism and capitalist indoctrination doesn't have a time limit.

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What does have a limit, however, is how much someone can understand the past with limited background. As discussed in the thread I linked above, people who ask questions on this sub very frequently, for lack of a better phrase, don't know what they're talking about. And that's okay! We expect that. It also gives those of use interested in education a great look at what misconceptions people have. Often, users will misinterpret the nature of historical governments, economies, militaries, and institutions as more like modern ones than they actually were. We like to think of the Roman government as comparable to our own nation-states, at least in the sense that "it's a government," but that's a shockingly limited comparison to make. The further back you go, the more "worldbuilding" you have to provide for things to make sense. A curriculum can safely start in the 1600s because that's when the direct roots of modern institutions were laid, and when history becomes "global" and not a series of case studies from different regions.

Consider, for instance, having to teach a 15-year-old what the word "mercantilism" means and why it's important. The term perennially reappears in textbooks, and just as often is difficult for kids to get. It's not that they don't understand why maximizing exports or accumulating wealth would be an attractive policy- quite the opposite. It's that that is so obviously a basic function of government that they don't understand why all of a sudden in the 1500s it would be important. And so you have to talk about the development of accounting, of legislative bodies capable of tariffs, of national exports even being a concept at all. It's difficult to conceptualize national economies as a high school student, but economies that lack all the things we know to make up an economy? How do you do that?

And so you pick your battles. As much as I would love to see more early history being taught, I also recognize that the more divorced an era is from our contemporary world, the exponentially more challenging it is to teach appropriately.