You're too useful to remove, you may not get attention for corrective measures because of fear of the person leaving.
You are often too swamped to effectively get work done smoothly and in an expedient manner.
You become a single point of failure (bus factor 1) so that when you take a vacation / hit by a bus, the company suffers significantly.
A common personality trait of these types of people seem to be self reliance / control seekers. When they own that authority of work, they are less likely to accept delegation to get them out of their woes.
In general, these types of employees may end up being labeled super stars, but they're also potential albatrosses that could significantly wound a company if not properly managed.