It might just be that if there’s an OpenSource option they prefer to use and support that, no need to have a threat to use something open.
Alternatively, maybe they want to create memes with their own personal images or text that they rather not upload anywhere. The reason for why they prefer some OpenSource option, maybe something to selfhost, is irrelevant to the question by OP.
What’s your threat model to need a quick online image caption to be opensource?
Why does it need to be proprietary?
It might just be that if there’s an OpenSource option they prefer to use and support that, no need to have a threat to use something open.
Alternatively, maybe they want to create memes with their own personal images or text that they rather not upload anywhere. The reason for why they prefer some OpenSource option, maybe something to selfhost, is irrelevant to the question by OP.