JK stays busy doing lots of things. Part of his Hunter's Hope duties include traveling to various state legislatures to implore them to include Leukodystrophies as part of the standard newborn screening process. There have been many advances in the treatments since HH was founded but only if the disease is detected at birth or shortly after. Still, most states don't require it to be tested for so often times the child is still diagnosed too late to benefit.
If you follow JK you'll notice he goes on a lot of hunting trips. Many of those are actually fundraisers for Hunter's Hope. Places around the country like hunting clubs who want to raise money will often raffle off a "Spend The Day Hunting With Hall Of Famer Jim Kelly" grand prize. They give HH whatever the agreed amount, JK goes hunting for a day with the winner, and the club keeps the rest of the money as their fund raiser.
And many of those celebrities that come to JK's annual golf tournament for Kelly For Kids also have tournaments of their own which JK is obligated to attend throughout the year for reciprocity's sake.
Hard to get into coaching and do all that too.
And besides, great players rarely make good coaches anyway.