At most attractive or at most fertile?
If most attractive, then arguably many men are also at their most attractive in their 20's before balding etc. hits - so why wouldn't most older women want 20 something men in the same way?
Because men are more looks oriented?
Because men, as I've said above, are on average more objectifying of the opposite sex and what they want/feel is not really considered?
Personally I think it is mainly fertility - and it doesn't have to be conscious i.e. with the intention of starting a family, it can be entirely subconscious/instinct.
Other points people have made about being more malleable, no baggage, status with other men/society etc. could be factors.
Anyway, I've had 2 relationships with older men (30 or more yrs, and 10 yrs) and both were disastrous.