The problem is more that many of the open source game engines aren't complete: features are missing, bugs not present in the original, etc.
These sort of projects tend to be pretty large and also fairly binary: either it's complete, useful, and playable, or it's not (yet) complete and it's unplayable and useless. There's a bit of grey area, but a lot less than many other projects. You can get away with "small implementation of X which slowly accrues features over time" with many projects, but a lot less so with reimplementation of games.