500 mm F5.6 L...why no such lens ??

HI
there is an obvious "gap" in the 500 mm prime area
i have always wondered why no one makes an affordable
500 mm F5.6 L type quality lens ??
I would have thought sigma would have ??

I'm talking about a high quality lens not the tamron 200-500 or 50-500 sigma style
I'm sure there's a market for it...i would buy an affordable one

Its obvious the new 200 -400 F4 L with built in teleconverter is not going to be afforable to most of us much as i'd like one !

can anyone tell me why no 500mm f5.6 L type lens around

PS before anyone says there a canon 400 F5.6L i know i've just sold mine

after 6 years..a great lens BUT not long enough !! (that why i want a 500 F5.6 L like it)

regards
Alan