Well, because nobody has standing to challenge it until they're actually drafted. If you don't understand what standing means, talk to a lawyer about it.
There were MANY high profile challenges to the draft in the Vietnam war and you MUST have heard about at least some of them - it was world news and your history text book will surely have mentioned them. And that's really the main reason why Vietnam was the last time a draft was used in the United States - people started winning those challenges, publicly, on the news, and Congress realized that striking down the law entirely was really going to be the next step. Dropping the draft, pulling out of Vietnam, and then rewriting the laws to tone down the draft, was Congress seeing the writing on the wall and giving the draft a de facto retirement but symbolically leaving the door open behind them; if a court took it away from them forever, it would be gone forever, and they know that if they ever try to use it again that's exactly what will happen.