Can't find compiled parameters values in the plan cache
sp_prepare doesn't include the compiled values for parameters, because it doesn't 'sniff' them. When a query is issued with it, cardinality estimates are made using the density vector rather than the statistics histogram.

I blogged sort of recently about that here:

Why sp_prepare Isn’t as “Good” as sp_executesql for Performance

If you read the blog post, some of the terms I've used in this answer are explained a bit more.