Exactly it's the same reason a majority of the ships we saw on the 24th century TNG era shows; were 23rd century models all seen in the various ILM Star Trek films.We probably also have to take into account real-life issues. Having to do all of the special effects from home may have forced the effects artists to use older digital models instead of creating new more time period accurate ones. We may just have to pretend that these are newer designs. Perhaps why they chose first season ships that were pretty much in the background (like the Hoover class) and could potentially be reused as a more contemporary ship type.
CGI models still take time and money to create, and stretching the budget is always a factor in film and TV production, so reusing models either physical or CGI saves money in the long run. for the majority of the audience nobody's going to care that some CGI model that was seen for two seconds in a quick shot or a from a long distance in a previous season episode of Discovery is suddenly seen in a more close-up shot, and is a ship of the 30th, 31st or 32nd century.
If its a model that's going to be used repeatedly (like the Booker character ship); then sure spending the a time and money to make a new CGI model is warranted; otherwise yeah, like the Berman era Trek series often did; reuse the models and extend the production money for use in other areas where it's better needed.
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