Stage kits come with a warranty, that’s why people get them
Not only that.
Since Stage Kits are a Factory Authorized item, it means a couple things:
The kit has been thoroughly tested enough and de-tuned enough to make them feel confident it won't result in warranty repair.
The tune is soft enough to work and not damage the full spectrum of variations of cars in which it is installed.
I will assume DC has access to all the secret code and makes sure it all works happily together.
When a problem arises with the car, the dealer will likely be able to figure it out because all the info about the DC Stage tune is on record and if necessary DC can be contacted for assistance.
If you have a Frankenstein, your options for analysis and repair are limited, which can be a real problem for a non-technical owner.
A prospective buyer may be very hesitant to purchase a Frankenstein, but may likely be much more open to a Factory Authorized Stage.
Yes, an aftermarket tuner will probably give you a faster car for much less $. But there is a lot involved in providing good feedback to the tuner for final adjustments. If the tuner makes adjustments based on feedback, then you later do something different (e.g. tune at the dragway then do a Texas Mile) you are taking a great risk.
The Stage is low risk/low reward, like CD's as opposed to the market. I like the market for $, but I like safe for my HC.