Going and looking back at a few posts because i was curious about it.
Prize Pool - 2018
Initial prize pool is $ 2,250,000 USD (approx. € 1,950,734 EUR)
12.5% of sales from Championship Kha'Zix skins & Championship ward skins is added to the prize pool.
Another 12.5% of sales of the aforementioned skins will be shared between all teams at the event.
In the event that a tie cannot be broken between Group Stage teams, prize pool money is averaged.
Approximate final prize pool is $ 6,450,000 USD
$ 6,450,000 US Dollars are spread among the teams as seen below:
Sourch : https://lol.fandom.com/wiki/2018_Season_World_Championship
The proceeding years, they didn't add on the sales money. Apparently the in client sales were left behind closed doors, unlike the previous years.
edit:
Prize Pool - 2021
Vietnamese teams will be given compensation, equal to 2.1% of the prize pool, to account for them not being able to attend due to travel restrictions.[3]
Gambit Esports will also be given compensation, equal to 0.6% of the prize pool, as they traveled to Shanghai as a backup team for LCL's Unicorns of Love, in case they wouldn't be able to attend the event.[4]
As a result of these modifications, more than 100% of the prize pool will be paid out this year.
Figures are based on the minimum guaranteed prize pool of $ 2,225,000 USD. A percentage of the revenue from designated in-client sales is yet to be added to this amount.
$ 2,225,000 US Dollars are spread among the teams as seen below:
https://lol.fandom.com/wiki/2020_Season_World_Championship
So there is a reason, just unfortunate we dont know the full number, other than the base purse riot gives.