For me, the reason I prefer TikZ is the same reason I prefer LaTeX: Preserving the original source code of the document is as important — if not more important — than preserving the compiled document.
I assume that every document I write will need to be edited, and this hasn't been too far off in my experience. I use LaTeX because it lets me express my document in a high-level, semantic, easily-editable manner. I can tweak the formatting, layout, and even structure of my documents by tweaking macro definitions. It's programming for documents.
I take the same approach for diagrams. Having the diagram is great, but I don't just want the diagram — I want to preserve how I created the diagram in a high-level, semantic, easily-editable manner. The diagram will need to be tweaked, and if all I have is an image artifact — even an SVG artifact — I can't make fundamental changes to the formatting, layout, and structure of the diagram without essentially starting over.
Tools like Inkscape are better than simple drawing tools like Paint, because at least they let you save and tweak the definition of the diagram. (This may include the draw.io tool you mentioned. I've never tried it, so I don't know.) But they have the same problem that word processors like Word have — they tie formatting and structure to the content. Inkscape and Word just don't have nearly as powerful tools of abstraction as TikZ or LaTeX, respectively. TikZ, like LaTeX, is a programming language.
It's been pointed out elsewhere that, since modern LaTeX implementations like LuaLaTeX and XeLaTeX support system fonts, it's easy to have a tool like Inkscape use the same font choices, but there are additional benefits beyond simply using the same font. For one, TikZ will automatically use the same font, so I only have to change it in one place and it can't get out-of-sync. Also, the font is just one variable of style — there's spacing, widths, enabled ligatures, and that TikZ can use LaTeX's math mode.
All that said, TikZ is complex, and I often do have to read a lot of documentation to figure out how to express what I want in TikZ — much like I do in LaTeX! They both suffer from the same complications and difficulties of macro programming, and TikZ has its own peculiarities with its key-value configuration system. It's a trade-off, and sometimes it's very tempting to use a WYSIWYG tool to create diagrams. But I generally stick with TikZ because I know that, once I've put in the work to create the diagrams, it will be worth it for all the reasons I've described above.