Isn't Dr. Scholl's insoles just as good as a custom orthotic?

No.  In fact, many people try these solutions before they come to see a Podiatrist and often find the "off the shelf" inserts inadequate to treat their real and substantial problems.  Factually speaking these generic type non prescription inserts are widely available because there is a definite problem and an ultimate need to handle the ever growing percent of people who suffer from numerous aches and pains from their feet to their backs from walking on the hard and unyielding surfaces that we work and play on each and every day. 
The point here is that the generic "supermarket or drug store" inserts do not significantly change, alter or realign your feet into a more functional and subsequently therapeutic position thereby not allowing you the substantial pain relief and restored function that your custom total contact orthotics provides.