It's just rude to do so, and there are some technical issues with doing that as well (such as crawling admin panel which might trigger backend alarms/security alerts). Google also doesn't have a legal monopoly on crawling, only a natural monopoly thanks to a lot of websites independently choose to only allow Google and Bing because of the many issues with third-party crawlers (eg. crawling all pages at once, costing money/slowing down the site[0]).

0: https://news.ycombinator.com/item?id=26593722