As a mathematician who has attended several philosophy lectures and seminars, I think this depends to a great extent on the discipline. I have never seen a mathematics talk delivered seated, while most philosophy talks I have seen were given by a seated speaker; occasionally, and particularly for bigger lecture rooms, the latter have been given by speaker standing at a lecturn. This most likely stems from the fact that in maths there is typically a lot of detail (notation, definitions, assumptions, results) to be stated more or less precisely and referred to later by the speaker or audience. Much writing or referring to slides follows from this requirement, which is not easily done while seated and facing the audience.
On the other hand in my experience of philosophy talks, the speaker typically reads or talks around a set text such as a written essay, so there is little if any need to write on or refer to a blackboard or projected slides. This leaves the speaker with the option of being seated while giving the talk.
I have never detected any suggestion that a talk was considered less serious or professional for being delivered seated.