Why Yahoo doesn't have an AdSense like program

.. it's because they compete with their customers

[blaze]

Because Yahoo is a portal and has a lot of content, they do not want to have a program which would go out and fund other people to build content that competes with their own.

If they had an AdSense like program, it would be more viable to build a website which competes with Yahoo because you'd have ready access to the same source of advertisers that they do.

They do have an contextual advert program with the big content sites, but that's because they are viable, AdSense or not, so Yahoo has no problem partnering with them.