I had two friends who did about 1/3rd this distance at about the same pace. In an era before folding beads, if memory serves I think they changed their tires out before day one, carried lots of spare inner tubes, but ended up carrying spare tires figure-eighted long before they hit the Continental Divide. The roads in our county were exceptionally good. That alone made our organized events pretty popular, but the organizers were also really good at what they do.

Going across the country you're gonna be hitting way more counties and states than you would even as someone who travels for cycling events. More importantly you're also not on a curated ride. Our club was top twenty and people agonized over the relative safety of certain back roads. Pavement quality, debris, traffic, blindspots, pets. Sometimes they got it right, sometimes wrong. It was an iterative process and some segments were only ever tried once (or twice if people forgot why we said no before).

On a cross country trip you don't have locals curating for you, so it's going to be more wear and tear.

In America, there are curated maps and routes by groups like the Adventure Cycling Association. Sure, there won't be members living along your whole route sweeping shoulders clean of debris, but they do a lot of work of trying to find routes that are as safe as can be, have good overnight stops, etc.

When doing long tours you want to replace stuff before they break. For instance, I did several tours between 3000 and 5000 kms. I might have gotten away with not replacing the drive train between tours, but spending that extra cash (especially when on the more budget friendly end, like me) is money well invested.

Also, with heavy load (bike+luggage for me was usually in the 35-40 kg range) and more riding in rain than I would do when not on tour, leads to increased wear.

My worst failure was a cracked rim on a rear wheel that I should have replaced before the tour. It had started to wear down due to bad breaks. It was still rideable for another 100k to the nearest bikeshop (this was in southern Spain).

The added weight of a touring kit is a big factor.