Is there any physical limitation to cell carries sharing the same frequency?
If two adjacent cells share the same frequency, they will interfere with each other.

Generally, frequencies are reused in non-adjacent cells, to give a chance for the distance to attenuate the unwanted signals to a manageable level of interference.

With proper design of signals (OFDM, with cyclic prefixes to mitigate multipath), you can achieve an SFN, Single Frequency Network.