If you don't have an "officially recognized" UA string some websites will assume you're a scraper/crawler/bot/ and rightfully so.
That's just a heuristic of the times. Soon bots will be much smarter than that.
They already are.
If I was ever trying to scrape a website that blocked my user agent I would just set the user agent to chrome on windows. Every library I have used supports setting whatever UA you want.