The former FIFA VP always maintained his innocence in an election bribery scandal, swearing he was the target of an American witch hunt and "would be fully exonerated by any objective arbiter." So then why quit?
Why join FIFA in the first place? Same answer. Money. FIFA wonders why we don't think they take corruption seriously, and then go ahead and have rules like "if Jack Warner quits, we can't investigate him anymore." Since Warner's resignation means he's no longer under the governing body's jurisdiction, he's de jure innocent. This despite "overwhelming evidence."
And what's more, he gets to collect his FIFA pension, reported to be about $37,000 annually until he's 96 years old.
The most important question here is, what's the incentive to be a clean ExCo member? If you don't get caught, you're well taken care of, and if you're caught, you're still well taken care of.
No, wait, the most important question is, how do I become an ExCo member?