It takes three years to teach
Splicing is not difficult to understand. You cut out what has failed, and you let in new timber that matches the old in grain, moisture and profile, with a joint that will not open up when the house moves. Understanding it takes an afternoon. Doing it well enough that nobody can find the joint takes about three years.
That is why most firms do not do it. It is quicker to build a new sash, and a new sash is easier to price. The skill has been thinning out for forty years, and every house that gets replacement windows takes a little more of the reason to learn it away.
There are five of us. Two are still learning. That is the honest reason we are booked five months ahead, and why we would rather turn work down than take on more than we can do properly.