Management Mindset: As long as the employee is not complaining, why throw more money? Besides, they can't be "that valuable and hard to replace".
I have even seen management who don't even bat an eye or goes extra mile to hire some 5-10x more expensive(even compared to market value) limited time contractor but zero interest or energy in putting up a fight to give a fair pay bump to one or more severely underpaid employee(s).
The tainting is a mind game: an employee suddenly hears from their friend or random recruiter about some lucrative offer(usually at least 25-100% raise in pay), tries out their luck and lands an offer, but now manager(assuming a sensible person) needs to put a fight[1] with upper management to bring that same bump(or comparable).
However, the relation is now tainted: employee now feels that injustice had been done and there is no guarantee that in future further raises/promotions will not come easy[2] and the manager might be looking into replacing them soon. The manager is now also sad that, they got under pressure by a leveraged employee+upper management will push to de-leverage such assets(yes a term managers use), and seek to replace such expensive assets soon. Now, both sides are running on bad trust and it gets immensely uncomfortable, hence the employee leaves anyways or get replaced by some shiny new hire who will command more than the original employee being replaced attained at their level.
[1] This is often difficult, unless your manager is politically influential, because once you have neglected an employee too long, adjustment can be bit high, it is now difficult to justify, how suddenly a particular employee became so much valuable out of the blue. Human mind is wired in such a way that, slowly ascending a hill feels less tiring compared to ascending steep jumps, as the latter takes more energy(or in case of employee big pay bump).
[2] Even though the adjustment being a correct valuation, any raise/bump of significance also brings expectations of more responsibilities and higher performance which is the same ol' undervaluing again in action.