Because it's not a qualitative analysis of the picture. It's an off-hand remark that would be better served as a comment.
"[This thing] in the picture is bad because [some reason]" has merit as a flag "What is that?" is not helpful
Then you should read the flag as a whole. It is also not helpful if one person only focusses on one part that is not important but ignores the part that really is important (i.e. the second sentence here). That said: If the flag is still descriptive to some extent in the end, then one shouldn't dismantle one part that is a bit sloppy. I personally have more problems with the word "ugly", but as long as "sketchy" stands there, I consider this flag as ok. One doesn't need to agree of course :P.
I'm not arguing against the flag as a whole, dammit. I'm not saying the first part invalidates the entire flag.
I'm saying the first part could have been excised from the flag and we would have lost nothing.
So you are making a fuss out of nothing then. It's not like a flag can be edited afterwards, even by a moderator, so that it only shows the "relevant parts".
To be perfectly honest, I don't think anyone reads that except for the people that edit it.
That doesn't really matter. If that accumulates, then the mods could write that user a message to stop writing such things that have nothing to do with the flag per se. Since it doesn't matter if it's read, but is has to be there. Just like the law^^.