It’s generally the result of them starting as personal or family businesses, so initially your brand is your name. Then the business grows so large the relationship inverts. It does not have much to do with germany really, it’s common the world over.

For example, just from US car brand, you obviously have Ford, but also Olds (Ransom E.), Chevrolet (Louis and Arthur), Buick (David), Chrystler (Walter), Dodge (Horace Elgin and John Francis), …

And I can’t fault a German industrialist from 1910 for not English-proofing his business.

It's not the lack of English-proofing, it's the connotations that this name has for Germans: "Sepp" is a very common and not really glamourous first name/nickname, and "frickeln" means tinkering or fiddling around with something (usually with the negative sense of tinkering rather than fixing it properly).

How long has it been like this though? The meaning of words change surprisingly quickly, but not as quickly as their second-order connotations.

Fun fact: It was originally legally mandated that if you form a business, it just be named after you.

I don't see the connection to German founders there, as many small / medium sized businesses like the one you linked are called after their founders everywhere (plumbers, printers, electricians,...)

Many do. The founders of Lidl bought the name from someone named Lidl. They were named Schwarz.