Annual inspections would suck large. Glad we don't have those. The HF trailer may get used ten times one year, none the next and three times the year after. Annual inspections would be the end of it and I would just rent if I needed a trailer. Another trailer lives at the cottage and normally travels less than 1 km /year to get the boat in and out. It would be annoying to pay to get that inspected. Like you, I look it over before I start moving to see if there is anything I am concerned about.You can even argue that the homebuilt trailer should go through a more rigorous inspection vs. a kit or a pre-manufacturer trailer.
A company that is in the business of making trailer should and most likely is well aware of the regulations it needs to adhere to as well as proper fabrication methods along with weight limits etc.
Someone that purchased a welder from CT or PA along with some stock metal, may or may not know anything about proper welding and limitations of said metal.
Without an initial inspection (or annual) who is to say that the trailer that was built while drinking a 24 of your favourite brew is worthy of being on the road.
Also, a lot of of these homebuilt trailers are seldom used and often sit outside in the elements detreating on someone's yard. Years could pass between uses and chances are that there are parts that would most likely need to be replaced. (i.e tires).
The intelligent person will do their own inspection and address issues as needed. But the reality is that unless enforced (annual inspection) these trailers will often end up on the road in not so road worthy condition.