This is more related to why there’s no legal pathways rather than “why deport”, but I could think of a few reasons that make it difficult to improve legal immigration:

• Bureaucracy in some countries (notable example being the US) makes it slow/difficult to pass new laws
• Since immigrants (legal or not) can’t vote, laws regarding improving immigration pathways aren’t usually popular among the voter base
• There isn’t a country in the world that has figured out how to “solve” the immigration problem, so there isn’t even a good reference point
• There is a global rise in right-wing and sometimes far-right sentiment, which are often targeted at reducing immigration, which makes improving immigration pathways even less politically appealing

I guess if legal immigration isn’t really an option then deporting someone might just seem like the “natural”, lawfully correct thing to do. I don’t exactly agree with this but I think that might be what’s happening