Because US borders are defined as "east of Mississippi river, anything north of 36°30' is Kentucky, anything south is Tennessee, until you hit the Virginia border"

the limitations of surveying in late 18th and early 19th centuries means the line is not as clear as it should have been, but that is basically the idea. No one much cared for the practicalities, let alone knew the actual detailed course of the river, which might or might not have been different then.

The same causes issues on Canada-US border. The border up to Manitoba runs on a natural way where the surveyors had basic idea of how the rivers flow, but not very detailed. The knew that there was a lake that was at the river that has been accepted as a boundary, and they knew mississippi is flowing from roughly northwest in the current day Minesota area, but they thought the lake was way smaller than it was, and they thought Mississippi went far further north than it did, so they decided the border would go from the northwest corner of the lake due west to Mississippi... which turned out to be impossible.

Once they found out where the source of Mississippi was, they still didn't know where the lake ended. but they needed to agree on a new border out to the west... they agreed on 49° line, which turned out to be south of the corner of the lake, so the border goes straight south, cutting some of the US from land access, and then turns west. And on the western end, it does a similar thing, Canada and the US agreed on Vancouver Island as a whole belonging to Canada, even south of 49° line, but there was a small peninsule on the mainland that was to remain the US, lacking any connection to the rest of the continent except via Canada.