Those lathe milling attachments only existed because prior to the 80's when manual machines started getting dumped out of machine shops for relatively little money to make room for CNC's a home user really couldn't afford a decent mill and in most cases lathes that were intended for home use rather than industry.
So they made affordable milling attachments to fit on the affordable home use lathes but none of it is very good.
This is a good point. To highlight this I like to look at the historical prices of Atlas / Craftsman machines.
Atlas kind of filled the niche of the current Chinese machines, basically targeting the light industrial and hobbyist market. Sears sold Atlas machines rebadged as Craftsman from the 1930s into the 80s.
In 1960 Sears offered four machine tools made by Atlas, a 6x18" lathe, a 12" lathe with 24" or 36" between centers, a 7" shaper and a small (4.5x18" table) horizontal mill.
The 6x18" lathe cost $175.50
The 12" change gear lathe cost $348 (24") or $389 (36"), a cabinet added $125, and a quick change gear box was an $86 option.
The 7" shaper cost $550
The Horizontal mill cost $495
These prices were without a motor. A good 1/2 hp motor suitable for the above cost $46.95
They also sold the milling attachment for 6" cost $43.20 (includes milling arbor), and the 12" cost $51.75 (includes milling arbor).
So if you bought the top of the line 12x36" Craftsman lathe, with cabinet, QCGB and best quality (for Sears) 1/2hp motor you had spent $646.95 + tax (about $6700 in 2023).
The mill with motor would cost $541.95 ($5620) so almost 85% the cost of that lathe, and you still need a lot more tooling to really use the mill. or you can spend about 1/10 the cost to get a milling attachment.
The Atlas / Craftsman mill, is honestly quite small and really more suitable for somebody that would be happy with the 6x18" lathe which makes the cost an even bigger hurdle being nearly 3x the cost of that lathe.
Benchmaster is another small mill of the period, offered in both horizontal and 2 in 1 horizontal / vertical mills. They were priced about the same as the Atlas / Craftsman mill. In 1960 these cost $475-510 depending on model.
Now you could get a Clausing 8520 vertical mill, for a mere $905 (1958 price, or $9600 in 2023).
(Edit just for fun I looked up the cost of a Bridgeport mill)
In 1964, a Bridgeport J head mill cost $1530 (about $15,000 in 2023 dollars), so probably just out of reach for most hobbyists.
The vintage milling attachments are more sophisticated then the import milling attachments, again likely due to the appearance of relatively inexpensive milling machines. Hard to sell a modern milling attachment for $500 when you can get a small mill for about the same price.