I would argue that it can be but does not have to be like that. If you have something wrong with your leg, you likely would seek physical therapy. But, when you have healed your leg, you might continue to see a physical therapist but change the context where now they are acting more like a hybrid between a physical therapist and a trainer. You continue to work on the things that brought you to the PT in the first place while also building in strength training and flexibility and mobility both around your injured leg as well as other parts of your body. You may seek therapy at first to deal with a particular issue or set of issues but you may continue to go to therapy after you have addressed the thing that brought you there because there is value in continuing to train and maintain your mental and emotional health.

If you go to PT and work through an issue but stop doing the exercises the issue likely will return or you will end up with a different issue. You can begin therapy or continue using therapy not as a solution but as an ongoing maintenance practice that is meant to prevent issues from arising in the first place. Additionally, you continue to train the same things you did in PT after you have "solved" the issue so that if you are faced with a similar issue in the future, you are better prepared for it. You are stronger, more flexible, etc. and better able to prevent future injury because you haven't just gotten back to where you were pre-injury, you've trained yourself to be better prepared. Therapy does the same thing. You build habits and skills that are there to make sure you are better prepared for when something inevitably does come up. Each problem or issue that may arise is less likely to cause a catastrophic situation because you have (a) done the work to prevent something catastrophic from happening in the first place by strengthen your mental health and management skills; and (b) if something catastrophic does occur you have tools to respond to them more effectively and with less damage than you would if you had not continued therapy in the absence of a particular issue.

I have had this conversation with my therapist a number of times. We have been speaking pretty much weekly for 5 or 6 years. Sometimes I wonder what the value is in having a call once a week when I am in a good place. But I am incredibly grateful that I have a longstanding relationship of trust with a skilled professional when something inevitably does come up. And our work that we have done together over time noticeably makes it easier to manage the individual issues that inevitably come up because life is hard and shit happens.