The reasons are:
- I pay Google per character for the translations so if users started habitually translating whole sentences it might be unsustainable without raising prices.
- Words and short phrases work better for flashcards and cloze tests in my opinion.
- The site is designed for intermediate and advanced learners who typically just need gaps in their knowledge filled in.
- I find that Google Translate often doesn’t handle large, complex sentences that well, so may lead to confusion, especially for the beginner users who are most likely to try translating whole sentences.