Well, yeah... that's because it's our time and skill and the privilege of making requests you're paying for, not your usage... would you expect to get a pizza with your choice of 3 toppings for free because you're planning to eat it at home with your friends?
While occasionally you may find people who are pure hobby creators, typically the work they will create free of charge is much smaller in scale than 3 continents created at a scale where a single country would be as detailed as a standard single page map. Usually the people I see doing free requests are doing small battlemaps with a simple pitch, like "a hill with bones and a tree on it by the water."
For reference, the approximate scale you're asking for (assuming 1 inch to 20 miles is what you mean by 20m) is 1:1500000
Here is an example of a map of Germany at that scale.
https://upload.wikimedia.org/wikiped...TK%28AV%29.jpg
This is a map of South America at that scale.
https://upload.wikimedia.org/wikiped...C_91685524.jpg
Any single one of those countries would likely be suitable for an entire epic length campaign to take place in and never leave.
The scale you are requesting would mean 3 continents the size of Earth continents would be an absolutely enormous map, probably have to be split into at least 3 maps, taking hundreds of hours of work. The map you linked is 1 inch = 100 miles and in a medium high quality style, and I'd estimate around 30 hours of work would go into making that map, so for that continent I'd estimate a starting rate of $300 to cover nearly a week's work of work, and for 3, well, I'd be quoting $800 to cover 80 hours.
If you want a free or cheap map, my suggestion is to pick a premade map and ask if you can use it for your home game, pick a cheap premade map from someone's Roll20 shop
like this one my client allowed me to release on the shop, grab Wonderdraft + Anzhc's mountain assets and DIY, or just run your campaign on these official DnD material maps from a module that includes a high res copy.
But you're welcome to hold out for a hero if you like. I just feel it's unlikely given the quite large scope of your request.