To be fair, Disney has been casting its employed acting talent since forever ago. (Even the late voice actor for Mickey started off as a custodian for the company.) They have that pool of talent close by, and I imagine they have predetermined rates to pay them for the work, so why not go with it?

On the other hand, using people who are seasoned voice actors doesn't always make for the best choice. After all, the voice for your iconic character above was most famous for co-starring with a talking horse back in the day. ;)