Why can I not book these flights on Air France?

This excellent answer by @Calchas, while for a different route on AF/KLM, happens to have the answer for your route as well - for whatever reason, they don't allow origin open jaws where the origins are in different countries.

I looked up Air France's fare rules for a first class flight from CDG-KIX, and they include the following (among many other rules). I'm not sure if these rules are easily accessible for the public at large, though.

OPEN JAWS

FARES MAY BE COMBINED ON A HALF ROUND TRIP BASIS
-TO FORM SINGLE OR DOUBLE OPEN JAWS WHICH CONSISTS OF NO
 MORE THAN 2 INTERNATIONAL FARE COMPONENTS AND THE OPEN
 SEGMENT AT ORIGIN MUST BE IN ONE COUNTRY. THE OPEN
 SEGMENT AT DESTINATION HAS NO RESTRICTIONS.

While user3761894's answer is unfortunately correct that AF will not issue such a ticket, there is one way around this that should be much cheaper than booking as two separate one-way flights: Book a round-trip from CDG to KIX and then book a separate one-way flight to LHR on a new reservation.

The downside to this option is that, with it not being booked on the same ticket, they're technically not responsible if a delay causes you to misconnect at CDG, but the CDG-LHR flight isn't that expensive anyway, so it's not that big of a risk. Plus, if you book Air France for both flights, they'd probably be willing to reschedule you anyway if a delay on your KIX-CDG flight caused you to miss the CDG-LHR one, even though they'd not be legally obligated to do so. Especially if you explained that you were simply not able to book them on the same itinerary because of the open-jaw fare rules mentioned in user3761894's answer.

Looking at a random date in February 2019, the CDG-LHR segment would be $135 with a stop in AMS or $170 direct if booked through Air France. Of course, the AMS-LHR segment would be operated by KLM if you booked one of the options with a layover at AMS.

The Air France flight from KIX to LHR (Osaka to London) may only be available on a limited schedule, or a code share since most international carriers need to fly to or from their home country. Since you want to do a stop over in Japan and come from and go to different countries it would easier to book the flight on an Japanese carrier like ANA or JAL.