Like a computer, the brain allocates it’s capacity to tasks in order of highest priority. Empathy is not so high on that list.
Air, water, food, shelter are at the top. Emotional flooding and trauma-management are near the top. Preservation of identity and world-model (psychological homeostasis) are not far below that.
Cutting to your question, non-empathetic people are very “busy inside” … all of their capacity is devoted to self and they are still running short.
To increase empathy, they must learn less chronic and energy intensive ways to manage their own core emotional/psychological material.
When the mind has a feeling of being ahead instead of behind, spacious, free and indulgent, it will naturally allocate social processing to noticing what other people are thinking and feeling. Empathy will grow from that.