Because there is officially a feminine gender noun to address female actors -‘actrice’, but not an official feminine gender noun for female teachers. So, “je suis professeur” can  be said by either male or female teacher, but “je suis acteur” will only be said by a male actor, as a female actor will say “je suis actrice”. One of those areas where language in use is changing, but not yet endorsed/recognised by the Académie. 

See lesson : Job titles differ in French depending on whether you're a man or a woman (nouns)

Okay so now you say we are professors using the nous sommes would you say it's both a female and male? Not just males or females but both?

Nicole, the default grammar gender for mixed grammatical gender groups, including for people, in French remains the masculine - so ‘nous sommes professeurs’, is correct. ‘Nous sommes acteurs’ is also correct for a mixed group. 

There is increasing use of ‘inclusive language’ in French - a common example is ‘Bonjour à tous et à toutes’, heard regularly on radio and television nowadays ( ‘Bonjour à tous’ is still correct and covers both male and female if used). As with many languages, gender non-specific pronouns are used by some, and inclusive spelling is developing. 

https://www.france24.com/en/europe/20210508-fight-to-feminise-french-language-racks-up-losses-and-gains-in-latest-round

Thank you so much I think I understand it now