Most mechanics at the reputable shop are not smart enough to install a simple seal kit so it would make sense they wouldn't want to do it
You're missing the whole point.
When your reputation and/or customer's money is on the line, you don't take those kind of chances. If installing a $50 kit works then your a hero, but, what if it happens like my first attempt did and the new seal is nicked by the sharp edge of the dogs on the drive adapter and it has to be redone? If you'd done it yourself it's no big deal, just means you have to buy another seal and spend another Saturday fixing it. If it's done professionally it's a come-back and your customer is now out more time and money and you're expected to fix it this time no matter what.
Let's say I put the kit in for a total of $300, $60 parts and 4 hours labor and it's still leaking. You're in a lose-lose situation. Most customers will not accept a redo on the kit and then, when you throw out the $1300 price of installing a re-man unit, they think your padding that price to recoup the original job. This sort of thing happens often enough with bad parts that most GOOD shops will not chance it knowingly. In case you don't know it, if I bought a re-man vacuum pump from Cummins, installed it, and it had a bad seal in it and I had to R&R it again, Cummins will supply another pump at no charge but they won't pay a dime to reimburse your labor costs.
Being a professional mechanic or running a GOOD shop is not about saving you money, it's about putting out dependable, good quality work while making money.
Fast, good or cheap. Pick two.
Good + Fast = Expensive
Good + Cheap = Slow
Fast + Cheap = Inferior Work