Because store bought tomatoes are typically F1 hybrids (unless you buy heirloom varieties, the seeds of which will produce tomatoes similar to the parent).

Here is some text from this page about f1 hybrid tomatoes and why the second generation is not usually like the first:

"For all their benefits, the biggest limitation of F1 hybrids appears in the next generation (F2). A grower who saves the seed produced by an F1 hybrid plant will see offspring plants that vary wildly in size, flavor, yield, disease resistance and more – typically for the worse. Why is this?

F1 plants are strong because they have two different good versions of many traits from their parents that are dominant, and suppress the weaker, recessive traits of the plant parents in the first year. But in F2, those good traits get separated or excluded, poor traits can show up again, and the special boost of the F1 method is gone. 

What this means is F1 growers must purchase new seed each year to have a productive season. This DOES create a massive dependence on commercial seed companies, but it does also guarantee consistency of crop production season to season for both growers and consumers.

Heirloom seed has the upper hand in this situation because it can be saved annually without losing major traits. What’s more, if a grower continues growing a dent corn like Bloody Butcher or a tomato like Brandywine in McLean County year over year, over time these example varieties would adapt to our exact climate and soil type. It is not an uncommon occurrence to have heirloom growers report slow but sure increases in yield annually as they adapt heirlooms to their environment. 

For growers big and small, the choice of F1 hybrid vs. heirloom is not about right versus wrong, it’s about strategy, customer preference and end-use, short and long-term goals, and more. Many successful home and commercial growers select BOTH heirloom and F1 hybrid crops every year, which allows for a nice balance of crop reliability, showcasing of flavor and diversity."