Historically from a revenue perspective essentially all the tarrif revenue came from bulk imports (trucks, ships, etc).

It is important to also capture tarrifs from high value parcels, as for small but high value items a parcel can very much be a similar bulk import. (Picture diamonds. A moderate sized parcel full of them is very much a bulk import).

But at the same time trying to collect tarrifs on every parcel was historically deemed non-viable. Way too much work for too little gain. Especially since historically, the addressee of the parcel often ends up paying the tarrif, this requiring customs to communicate this to the parcel carriers broker, who must communicate it and collect from the end customer, who finally gives the money to the broker, who submits it to the government. Meanwhile CBP needs to store this package.

This whole process ends up just annoying your countries own citizens, and generated little revenue, so a de minimis exemption of some form was highly desireable. And to most politicians there seemed like little downside to setting it fairly large. Sure $800 is probably larger than reasonable, but it certainly means the average person would rarely ever need to interact with this process, and that was good enough.