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 the 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; however, see the Chazon Ish's answer, below.]
- (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.
Midrashim quoted by the Tora Sh'lema say that this is the source of "al tiftach pe l'satan" [which I guess means that they agree with the Tos'fos Yom Tov's own answer].
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. [This, too, is opaque to me. Perhaps he means the Tos'fos Yom Tov's own answer, or en m'shivin al hakalkala?]
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.
Abarbanel says he should make himself seem uncertain and not like he's smart. [This answer seems to me like the Mizrachi's teachers' answer, but I'm not convinced they're the same.]
In Esh Kodesh, the Piazetzner says that the homeowner is unable to see on his own whether the tzaraas is a good thing in disguise (so as to get hidden valuables) or simply a bad thing (a nega), so all he can say is that looks like a nega to him.
Rabbi Avraham Yitzchak Shayn of the Yeshiva of Staten Island writes in his book, Birkas Ish, that the homeowner, if he were sure that he were not guilty and that the tzaraas were so he could get hidden valuables, would be glad to announce he has a nega; on the other hand, if he thought he might be guilty and that the tzaraas were a punishment, he would only say it's k'nega; so the Torah is telling him that he should think he might be guilty and examine his deeds.
The Malbim, if I understand him correctly, seems to be saying that the problem is one like more halacha bifne rabo.
Chazon Ish says that if the homeowner is a talmid chacham then the kohen will need to rule like the owner said, that there is, in fact, a nega, since, although only a kohen can rule on a nega, anyone can check it. (This works, he notes, according to those who hold a homeowner can check a nega on his own home, which he says is a matter of dispute among rishonim.) And if the owner is not a talmid chacham (or if he saw it at night or under other circumstances that don't count) then (a) saying it's a nega is lying and (b) one shouldn't begin talking by saying something bad.