They’re not. Here’s the beer history lesson of the day:
Pilsners were developed in Pilsen, a city in western Bohemia in what’s today the Czech Republic. It was kind of a revelation at the time- until then, beers were more like the English Pale ales, Munich Dunkels or German Alts. Essentially they were all somewhat dark, and most had a fair degree of yeast character, except in Bavaria, where the original lager yeasts developed.
Combine the yeasts and Pilsen’s extremely soft water, and the brewers found that they could actually brew a pale beer. (the more alkaline the water, the more dark malt has to be included to get a good beer without water treatment)
Even at that, a “real” Pilsner wasn’t like Bud Light. They were still in line with their darker, funkier counterparts in terms of gravity and hop levels (IIRC 1.050-ish and ~40 IBU), but the yeast and water combined to make the new beers all about the flavors of the hops and grain.
These new lighter, clearer, more crisp beers took off, becoming the “default” in most places without a strong brewing tradition already in place, and even making inroads into places that did have that tradition (Germany, for example).
This extended to the US; in the 19th century, German immigrants brought brewing, and in particular lager brewing to the US. People like Adolphus Busch (Anheuser Busch), Adolph Coors (Coors), Bernhard Stroh (Stroh), Gottlieb Pabst/Jacob Best (Pabst), Kosmos Spoetzl (Shiner Brewery), Joseph Schlitz (Schlitz), Friedrich Muller (also known as Frederick Miller) (Miller) and Gottlieb Heileman (G.Heileman) came to the US and opened breweries.
They mostly brewed adaptations of the traditional European style pilseners, with some changes made for local US ingredients, in particular six-row malt. In order to get a clear product due to the protein content of the six-row, they had to cut it with rice or corn. But the gravity and hop content remained the same. These were NOT watery beers.
The biggest single driver of the weakening of mass-market US beer was World War II. Because a lot of men were in the armed forces, the brewers aimed toward the working women of the era, and started making the beer weaker. They noticed that even after the war, as they lowered the OG and hops, sales would increase.
That’s how we went from a beer that was maybe 70% malt, 30% rice or corn, and brewed to an OG of about 1.052-ish with 30-40 IBU, all the way down to one that’s probably an OG of about 1.030, and probably around 12 IBU. In other words mass market US beer hasn’t always been this mild and weak.