Since Seven/Annika was just a civilian member of the crew with a science background (a civilian laborer, in other words) giving her a Starfleet uniform would go against precedent and protocol as to how people are on Starfleet ships.
Oh come one, under pretence of respecting protocoles/policy imposed by Starfleet, Janeway, at multiples times, allowed herself to make and break rules/regulations and precedents when it suited her well and examples are numerous!
About clothing issue, I remind you that she imposed to Chakotay and his crew to wear a standard Starfleet uniform to better blend into the already existing crew but allowed them too to wear a symbol which differentiated them from others who wore the traditional pips ; she let Kes and Neelix a total freedom in the choice of their clothes under the pretext they were just civilians.
Seven is another problem: she was maybe a civilian but she know what the word "discipline" meant, a remembrance from her 18 years spent in a Borg Collective ... even if yes, she took a great pleasure in doing only what she wanted and when she wanted. But right the moment, she trusted Janeway and some of her people, she began to follow orders. Plus, I will add that she had the knowledge to hold her position of Head of Astronomy Dpt and knew a lot of things in Engineering Dpt, an Starfleet uniform would have allowed to feel completly accepted as one of them.
Of course, she would have needed a special uniform offering the same advantage as the catsuits she wore but well, an uniform remains an uniform, right?! It is not so much the cloth in itself which counts but the feeling to be a part of something.
Besides, I note that in some fanfictions, Janeway ended to offer her one specially made for her and with pips, of course!