If all beings have had an infinite number of past lives and have encountered and practiced all types of teachings, why aren't we all already enlightened?
I think a couple of things need to be clarified first: When the suttas and sutras were translated into English some things lost meaning and/or gained new meaning. In reality, the Buddha didn't speak of an infinite
number of beings, but rather a number of beings that cannot be fathomed.
Also, when the Buddha spoke about no beginning he said the beginning cannot be found. He also spoke about no beginning and no ending
(try reading the Heart Sutra and "No beginning, no End"
by Thich Nhat Hanh).
This idea often makes newcomers to Buddhism say "well if there's no end, what's the point!"
, well take a look at the water cycle
. When we look at the water cycle we can't find the beginning, nor the end. But of course it had to have started somewhere, we just can't find it. This is what the Buddha is trying to say. The beginning of suffering can't be found and the number of beings cannot be fathomed.
This teaching is often hard to accept for a newcomer which is why you'll often hear people saying to leave it for now. The Buddha said
Conjecture about [the origin, etc., of] the world is an unconjecturable that is not to be conjectured about, that would bring madness & vexation to anyone who conjectured about it. http://www.accesstoinsight.org/tipitaka ... .than.html
Hope that helps,