Because that is the standard rule in English. The OED says:
Hence, in modern English, C has
- (1) the ‘hard’ sound
[k] before a, o, u, before a consonant (except h), and when final, as in cab, cot, cut, claw, crow, acme, cycle, sac, tic, epic;
- (2) before e, i, y, it has the ‘soft’ sound
[s]. In all words from Old English or Old French, final c is avoided: the
[k] sound being written k or ck, as in beak, meek, oak, book, bark, balk, bank, pack, peck, pick, rock. This is probably due to the claims of derivatives like meeker, oaken, barking, rocky, where c could not be used. Final c however is written in modern words from Latin, Greek, or other languages, and (of late) in the ending -ic, as in sac, tic, epic, critic, music, picnic. In the rare cases in which this c is followed in inflexion by e or i, it is necessary to change it to ck, as in physicking, mimicking, frolicking, trafficker, picnicker. When the
[s] sound is final, it must be written ‑ce, as in trace, ice, thrice, and this final e must be retained in composition before a, o, u, as in trace-able, peace-able.
- (3) Ci (rarely ce) preceding another vowel has frequently the sound of
[ʃ], esp. in the endings ‑cious, ‑cial, ‑cion, as atrocious, glacial, coercion (ocean). This sound (which is also taken by t in the same position) has been developed in comparatively modern times by palatalization of
[s]. In a few words from foreign languages, c retains the foreign pronunciation, as in It. cicerone
Which leads us to examples like colicky, havocker, picnicky, plasticky, panicking, picnicking,
panicky, magicked, colicking, picnicked, bivouacking,
colicked, mimicked, frolicked, picnicker, demosaicked,
garlicky, mimicker, havocking, bivouacked, demosaicker,
havocked, panicked, mimicking, frolicking, demosaicking.
Yes, you will sometimes see words like those misspelled without the protective k, but that’s like spelling the plural of bunny as *bunnys instead of as bunnies: it’s just plain wrong. We do not do things that way in English.