Why do tree seeds dont always reproduce cultivars characteristics

For a short anwser yes. But, it's like the lottery. Your chance of reproducing the exact characteristic well known are minimal. To be in context, bonsai enthusiast often praise a specific species for it's characteristics that adpat well to bonsai growing techniques. For example, small leaves and small internodes are quality sougth after.

You can find these specific characteristics present in all japanese black pine's (Pinus thunbergiana). But some growers have found a certain plant, that as even more pronunciated specific characteristics. From that plant, they have reproduce it by cloning technique such as cuttings or layering. This plant was attributed a name, wich is called "cultivar".

To continue our example, the common name of japanese black pine is Pinus thunbergiana, but a cultivar would be Pinus thunbergiana 'yatsubusa'. This yatsubusa is a cultivar of the japanese black being known for it's short needles and short internodes compare to the general japanese black pines. Another example of a cultivar would be 'Nishiki Matsu' which is known to offer a japanese black pine with a cork like bark.

But can growing bonsai from seeds reproduce these characteristics ? Yes and no. We say yes and no, because, you cannot reproduce 100% the same genetics as the known cultivar but you can reproduce another genetic sets that will have the same characteristics, ie: short needles or cork bark. Furthermore, you can produce an even more pronounced dwarf plant, or with leafed tree's, a new color of leaves perhaps.

Bonsai seeds, as in seeds from a bonsai tree dont exist. Growing seeds from a bonsai tree will not guarantee a bonsai. All seeds are tree seeds since a bonsai is fundamentally a tree. But some tree's have been found to produce seeds that will mimic the parent characteristic's. The yatsubusa tree seeds are one of them. Yatsubusa, which is japanese for witch broom, is a genetic disease caused by a fungi. When a tree is affected by this disease, it will produce dwarf shoots, and seeds produce will inherit the disease because it is genetics. Chinese elms, Trident maples and japanese black pines are three known species that have this yatsubusa disease.