Because it works, and nobody has stopped it from working.
This pattern shows up constantly and it follows a predictable script. The inmate asks for money. The person on the outside cannot send it, for whatever reason. Suddenly there are accusations, jealousy, threats to end the relationship. It feels like it came out of nowhere, but it did not. It is a pressure tactic, and the reason it keeps happening is that it keeps getting results.
Prison is a powerless environment. An inmate controls almost nothing about their daily life, and that loss of control is psychologically brutal for a lot of people. Money from the outside is one of the few tangible things that changes their daily reality, commissary, phone time, small comforts, and when it does not come, some people respond with the only leverage they feel they have left, which is the relationship itself.
That does not make it acceptable. It makes it understandable, and understanding it is actually useful because it tells you exactly what to do. If the behavior has been tolerated, explained away, or rewarded with money sent out of guilt or fear of losing the relationship, it will continue. The dynamic only shifts when the person on the outside decides what they will and will not accept and holds that line consistently.
You are allowed to be supportive without being a target. Those are not the same thing, and anyone who makes you feel like they are has already answered the question of whether this relationship is healthy.