It's amazing how much is going on in three sentences.
No one has to be a amp guru or designer to start a successful amp business. They do have to be a good business person with business administration education or experience. Lots of people can build, even design, a great amp, but only a few have the knowledge to successfully market the amp. Plus, beyond managing supply chain costs, overhead, wholesale distributor network, and marketing, with consumer electronics, there is a liability issue, if you want to be insured, you're products will should be UL certified in the U.S., CSA certified in Canada and are required to be CE compliant to even be sold in Europe (which is why a lot of boutique brands do not sell to the EU). If you are insured and your amp catches fire in someone's home,
To the original request...
Building an amp, designing an amp, and marketing an amp are three very different things. Building an amp is skilled labor based around best practices and safety precautions, and designing is mostly math on a chalkboard (that is also useful for diagnosing a broken amp). Very few amp companies use original designs, they are are streamlined or derivative versions of classic amps. And even the most "original" almost always start with a classic as the jumping off point. And it's amazing how many lauded amp brand are built like crap but sound great and are marketed well.