Data export is not that easy. I've looked into it for my own project. You have to determine which database fields you need to include in the export, what format those fields should be in, and then format of the export.
After you do all that, you need to code and test the export process. I estimated several days if not a week or two to get a workable system in place.
Edit: This is for a Rails project. If it was PHP, I wouldn't even bother thinking about it.
Not to mention the need to set up the actual page layout, markup, and related code... this is a PHP app so it's not that easy. Since I'm the only developer/everything else guy, my time has to be split between customer support, development of cashflow-generating features, and everything else needed to run a business. Maybe you're the "check out the Turing-test-passing AI I developed in a weekend" kind of programmer, but I'm not that good :)
I don't know about you, but if the requirement is simply "give me my data if I ask," I'm going to do as little as possible to get them that data, and then let them do what they wish with it from there—if it's not in a format they understand, then let them hire a third-party to write a converter; that's not my job. In this case, I think I'd just expose a single API endpoint that just spat out SQL dump statements.
After you do all that, you need to code and test the export process. I estimated several days if not a week or two to get a workable system in place.
Edit: This is for a Rails project. If it was PHP, I wouldn't even bother thinking about it.