Because "unwelcome sexual advances" is a spectrum from acceptable to unacceptable.In a situation where there is a lot at stake, socially savvy people generally want to know the answer will be "yes" before the question is asked.
Historically, when men were the primary breadwinners and women were expected to be homemakers, a man asking for a date from a woman he had determined was single was not likely to be causing big problems for anyone. If she said "No" and he dropped it, no drama would ensue.
This is not true where they are business colleagues and she has career aspirations.
I had an entry level job at a Fortune 500 company for over 5 years. I wanted to get a better paying job more in line with my education in the IT department.
One day, a senior programmer asked me for a date. I'm sure he had stopped to consider if he was in the clear and determined he was. He wasn't doing anything wrong and he wasn't going to get in trouble.
I'm equally sure he didn't stop to wonder how it impacted my career aspirations. It basically killed any hope I had of getting a job in his department. Simply asking me out closed doors for me.
I left the company shortly thereafter. I left for largely unrelated reasons, but that incident made it clear to me I could basically give up all hope of ever escaping the Pink Collar Ghetto I was trapped in because some powerful man at the company saw me as date material and never stopped to ponder how that framing might impact me.
In over 5 years, he was the only person I ever met at the company who knew what GIS was without me having to explain it. He never once wondered what my career goals were, nor what my unusual skills might do for the company. Nope. He just got all excited about the possibility of getting a date. Full stop.
This is the essence of why we are seeing studies like this one about the impact on women in specific, even though anyone can be a victim of sexual harassment.