Or, a less cynical take is that they really can have more impact by focusing on fewer charities. Another commenter posted Amazon Smile's Form 990 from 2018 [1], the most recent year on Pro Publica's website that has a full listing of all the charities that received funds.
The total PDF is 2121 pages long, and the list of charities starts on page 18. They made about $37.5 million in total donations that year. The top charity, the ASPCA, received $1.8 million that year (note that the ASPCA had total revenue of $267.7 million that year). But the main point is that the charity on the top of the second page, which is only about 100 charities from the top (ordered by total amount received) received just over $10,000, and the charity at the top of the third page received $5659. And there are still over 2100 pages of charities to go!
So it seems imminently reasonable to me that, rather than give a couple hundred bucks a year to thousands and thousands and thousands of charities, to instead consolidate that to fewer charities where it can do some sizable good.
1. https://projects.propublica.org/nonprofits/display_990/46262...