Why is the meta description in Google search results not the same as the meta description input into the CMS?

Google uses the meta descriptions on the site as "suggestions" only and they automatically create their own snippets to better match the search query. The full Google documentation on this topic can be found here: https://developers.google.com/search/docs/appearance/snippet

Available data suggests that Google currently re-writes meta descriptions for ~70% of searches. It is not possible to force Google to use the meta description provided by the site instead.