It's a matter of trust: rearranging someone else's words in a paraphrase or summarizing their ideas by precis is not a creative act. If you cannot copyright what you write, give credit! That is how trust in the academic process is built. Would you trust someone who continually ripped you off? Build academic trust and confidence in your own work by citing all material that is not "yours".
It's a matter of honor: Our social fabric and civilization are build on the concept of honor. We honor the past by citing earlier contributions to culture. We honor the present by respecting the work of others who contribute to the academic process. We honor the future by good practices in the present. Please cite all your research! Honor is personal. It has its basis in truth. Being truthful is essential to the academic freedom that guarantees our democratic governance.