Why ideal filter cannot realised practically?
Most typical examples of the ideal filters are the classical brickwall frequency selective filters (lowpass, highpass, bandpass etc) which are defined as ideal because of their impossible to realize ideally selective frequency responses.

Their frequency responses include exactly flat passbands, exactly flat stopbands having absolute zero gain, and zero width transition bands. These features make it impossible to realize them using either existing physical devices or finite time computable algorithms...

Assuming an LTI system model for such filters, then their impulse responses tend to be of infinite length and typically be acausal; another reason for denying their physical realization. Yet causality alone is only a concern for real-time processing of time domain signals. It does not prevent realization of such filters in offline procesing where the whole signal data with its past and future are already available. Yet even in such an offline processing mode the fact that they require an infinite length of data is the reason for preventing their realization.

Another point of view into their unrealizability is that input/output relationship of such ideal filters cannot be described by finite order differential / difference equations. In principle any filter whose input/output relation can not be described by a finite order differential / difference equation is said to be unrealizable; i.e., there is no finite time computable algorithm to implement it...