Biology and the Cell

Why does biology start with the cell and work upwards? Why explain biological phenomena in terms of cell activity, rather than cell activity in terms of the activity of larger systems? No doubt there is experimental evidence to support this approach, but I find it prima facie doubtful. In many other areas, we know that a combination of units is greater than the sum total of the units that comprise it. A basketball team is often better (sometimes worse) than the individual players that make it up; there is such a thing as “team” that has real causative effect on the outcome of a game and is not reducible to “players” or even “players + coach.” I suspect there’s some causative power at the system (organ, organism, whatever) level that cannot be reduced to the sum total of the cell activity.