Plants change. Setpoints move, equipment is replaced, operating strategy shifts, control systems are upgraded and tag names change with them. Every one of those breaks something in an analytics stack, and someone has to fix it.
In year one the builder is present and fixes things in an afternoon. In year three the builder has moved to another role or another company, the documentation describes an earlier version, and the system produces output nobody quite trusts. This is a common ending for a built industrial analytics capability, and it is organisational rather than technical.
The cost to carry is not the build cost. It is a standing allocation of engineering time, indefinitely, plus a succession plan for the person who holds the knowledge. Costing that honestly at the start changes many build decisions, which is presumably why it is so often omitted.
A bought system has the mirror-image risk, which is worth stating fairly: the vendor changes direction, is acquired, or ends the product, and the plant is left with a dependency it did not choose. The mitigation is the same in both directions, which is owning the data layer and the contextualisation regardless.