In imperial units, things always go by fractions of an inch. for some things, it makes plenty of sense: "I'll grab a 1/2" drill and a 1/4" drill. For others, not so much: "Let's use some 3/16 tube for this". Why 3/16" (0.1875") instead of 0.2"? It seems so much simpler. Perhaps this is a silly question that has the same roots as "why don't we just use metric?"
Fractions didn't win out in manufacturing. The vast majority (I won't say all because I'm sure someone is still building microwaves or something using SAE units) use SI units, which are metric. The last document I personally saw that used SAE, with the exception of service documents which will have something like [1.5mm (1/16")] or SAE for fasteners, was from 1973. As manufacturing has become more global it just made things easier for American companies to switch over to metric as well.
The reason that the average person still buys a 1/4" drill bit and service information still including SAE most likely boils down to a combination of there being no legal requirement to use the metric system even though American customary units have been outlined as metric since 1975, and the fact that it is difficult to get grandad to change the units he thinks in. Until SI unit use is legally mandated we aren't likely to see the consumer side of products and documentation being metric.
Edit: I forgot to mention the monetary motivation for the inclusion of SAE and the issue of obsolescence. If SI units were legally enforced tomorrow then everything that currently uses an SAE component would be immediately obsolete and would quickly become impossible to repair and maintain which would likely cost in the realm of $370 million according to NASA in 2015. That cost could increase or decrease depending on the length of the implementation period.
I also don't know why I forgot this originally, but it is also difficult because currently in the US the right to change weights and measures goes all the way down to the county level of government because following customary units is completely voluntary. I say this somewhat jokingly, but if a county decided to change their speed limits to bigmacs per bald eagle, and to use cubits in their building codes they can legally do that. It would require quite literally an act of congress to change this. A stretch of highway 19 in arizona has its distances measured in kilometers, just to prove the point.