This is because text is sent via the MMS protocol. The MMS protocol has a file size limit. When you send a video via text, it is first compressed and reduced in size to meet the MMS requirements. This compression reduces the quality of the video.
A better way to share videos is to save the video to the cloud and share a link from the cloud to your friends. For example, after saving a video to Google Photos, in Google Photos you can select to share the video with people.