For two main reasons.

First reason is the training camp partial guarantee, which isn't part of a players salary. (where the max is $26k as you pointed out) Think of this as a "signing bonus", where certain high profile prospects get a bonus before the season begins. In 2016-17, this raised the earnings for 21% of the G-League to a minimum of $46k. A top 10 HS recruit certainly would be part of this group of high profile prospects. Unless he gets screwed over by the G-League, say if they were to take advantage of his youthful naivety and potential lack of management. (which isn't really going to entice future HS prospects now, is it)

Our very own Jarrell Eddie made $175k in the G-League in 2016-17...just for a training camp payout.

Second, the addition of top shelf HS prospects may raise the overall standing of the league itself, which may inflate attendance, TV revenue, salaries, etc... A rising tide floats all boats.

Lastly, you compared making the NBA to "lottery odds". Clearly, a Top 10 HS recruits chances of signing an NBA contract are far greater than "lottery odds".

I'm not saying anybody is right or wrong, or a college education doesn't have value, but blanket statements like "19-26k!" or "making the NBA is akin to lottery odds!l" doesn't really hold much water in this case.