Because we can! 'As well' means the same as 'too' or 'also', and its correct position is at the end of a phrase. 'Though' at the end of a phrase means the same as 'but'. You can't put 'but' at the end of your sentence, but you can end it with 'though'. The fact that we have a choice of words that can go at different points in a sentence makes our language flexible. I hope that answers your question.