Tos'fos Yom Tov to N'gaim 12:5 gives the following answers, if I read him right:
- (from the Mizrachi in the name of his teachers) Since he can't be sure it's a nega until the kohen shows up, he shouldn't say he's sure.
- (from the Mizrachi himself) He should treat the kohen with derech eretz, etc. [The Tos'fos Yom Tov includes the "etc.", thereby referring he reader to the Mizrachi for a fuller explanation. I didn't check the Mizrachi or, indeed, the other people he quotes.]
- (from the Mizrachi himself again) So the kohen not be swift to judge it as tame. [I assume this means that he'll be prejudicing the kohen.]
- (from the Gur Arye) It's not a nega until the kohen says it is, and he shouldn't lie. (The Korban Aharon asks that that's not true: the kohen says it's tame, but it's a nega tahor even before pronounced tame by the kohen. The Tos'fos Yom Tov answers that "nega" in the vernacular means a tame one, so he'd be lying to call it that.)
- (his own) Because of al tiftach pe l'satan.
Rabbi Matis Blum of Queens, in his "Torah Lodaas" weekly d'var-Tora sheet for M'tzora 5771, suggests that according to those who say there never was or will be a house with tzaraas, "k'nega" makes sense, as it isn't, in fact, a nega.
Ralbag says it's because only the kohen can adjudge it a nega. [This is opaque to me, but perhaps he means the Mizrachi's teachers' or the Gur Arye's answer?]
Rav Moshe (in Darash Moshe) says it's so as not to say something bad.
Rabbi Chayim Zuckerman, in his sefer Otzar Chayim on chumash, quotes רמה״כ (?) as saying that because tzaraas appears on a house because of tzarus ayin, the owner's saying it's a nega is effectively his libeling himself.