Answer may be:

This happens because hip joints and lower back muscles have tightened over time. With regular practice, you can get more comfortable. Try using rolled blankets or towels for additional support under the knees.

With additional support, inner groins relax and make you more comfortable sitting.

Additionally, you can practice some asanas like child pose, toe touch, pigeon pose, and vajra asana which are helpful for leg and thigh muscles. If you have a medical condition that prevents you to sit cross-legged, avoid doing it and consult a doctor before doing it.