why is amplitude maximum only during resonance

Well Sing I spose you could go Google, but probly you've been there. Excite the resonant circuit with a single quick pulse for instance and watch the resulting waveform. You'll find it oscillates for a few cycles with a diminishing amplitude. But if you excite it at a different frequency its natural tendency is continually interrupted in such a way as to discourage buildup

Imagine a kid on a swing, when your slightest nudge forward will lengthen the arc, but many of those applied at a different rate would slow it

...while subsequent postings will probably tear me apart