I follow competitive running closely; this just isn't something that would happen. Running a marathon takes a huge toll on the body. Professionals typically only run two a year (although certain East Africans sometimes do one or two more), and they usually take an extended break after each one. Competitive runners never approach race effort in their training, especially at marathon distance — it's counterproductive to do that sort of damage to your body.
There's also a financial incentive to run your fast marathon in a race, especially for the runners from poor East African countries who are the only ones capable of running close to the world record. The prize money is typically several hundred thousand dollars if you include the bonuses for breaking the course record and the world record, finishing faster than certain time thresholds, etc.