He was born rich. His parents are medical professionals so they can afford all the training, airfares, lodging, expensive coaching, etc. to make his son become an Olympian which (I think) is probably more prestige than a so-so MD or engineer. I guess after achieving his goals as a top 10 table tennis player he will follow his parents path.
Thats true, but I think its not only the family money.
I believe KJ takes a good support both from official sponsors and casual sponsorships.
Here is his sponshorship request when he was in 3rd Bundesliga league:
"WHY YOU SHOULD SPONSOR ME?
I am 17 years old table tennis player, dedicated and passionate about promoting the sport in USA and being a good ambassador for my country. Some of my past accomplishments are: 1. Currently ranked #1 in the world in Junior Boys Under 18 in the March 2018 ITTF World Rankings 2. Being the youngest a thlete to represent USA at RIo Olympics in 2016 3. Being the youngest athlete ever to qualify for the WorldCup in Dusseldorf, Germany in 2014 4. Being the youngest athlete to represent the USA in PanAm Games in Canada in 2015 5. Two times US Nationals Men's Champion, 2016 and 2017 6. Current World Circuit Junior Champion 7. Currently live in Germany for training and play for Ruhrstadt Herne, a German Club in the 3 Bundesliga league. I am dedicated and disciplined and my dream is to put USA on the medal stand in Olympics one day. "
@OP
If you want to make money as a top player, you would have to invest your own money to get close to the described level.