I wouldn't say it's it's straightforward to convert between them, either. The Gregorian calendar was first adopted in 1582, but the UK and it's colonies didn't follow suit until 1752. Russia didn't convert until 1918, and Greece didn't switch until 1923. We're just over 100 years out from active use of the Julian calendar. So you have to know who the speaker is and where they are or where they are from to know what they mean.
Since these places switched at different times, they had to skip a different number of days. It was 10 in the 16th century. It was 13 in the 20th!
If you're reading a journal by a British man who lived in Russia through the end of the Great War and then moved to Greece in 1920, what does he mean when he's talking about a lunar eclipse he just witnessed?
If you are talking about a historical event from Egypt in 500 BCE, you would say "this happened in 500 BCE", not "this happened in year 26 of the 27th dynasty".
The distinction is important in the context of doing historical research, but it is no more appropriate to write dates in secondary sources using the Julian calendar than it would be to try to explain Egyptian history by spouting hieroglyphics at people.