Why most gymnosperms are tree like plants?

Why there's very little morphological variation in terms of architecture in gymnosperms as opposed to angiosperms? Why no grass like, forb like, weed like, or aquatic gymnosperms, with the exception of Welwitschia?

Many of these life forms are not entomophile, like grasses or seagrass, so I don't think the lack of flowering structures in gymnosperms is the explanation.