Using the Bottom-up and Analogus Estimating techniques

In the Estimate Activity Resources, we use the Bottom-up Estimating.
In the Estimate Activity Durations, we use only Analogous Estimating.
In the Estimate Costs, we use both Bottom-up and the Analogous Estimating.

With Bottom-up being more accurate, why would it not be considered in all the processes. Or say both Analogous Estimating as well as Bottom-up Estimating used in all processes and the technique being used as per the needs?

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Bottom-up estimate can be used only if you have clarity of all activities to be performed, secondly its more time consuming.

Depending upon the project context you will have to use the estimation technique. Analogous estimation can be used in many situation , say you are signing T&M contract with client you can use Analogous method to give ball park of cost. Likewise it can be used in other scenarios.

Estimation is tough in certain industries like Information technology , and still evolving due to frequent changes in technology and client demands.