Database tuning can be an incredibly difficult task, particularly when working with large-scale data where even the most minor change can have a dramatic (positive or negative) impact on performance.
In mid-sized and large companies, most SQL database tuning will be handled by a Database Administrator (DBA). But believe me, there are plenty of developers out there who have to perform DBA-like tasks. Further, in many of the companies I’ve seen that do have DBAs, they often struggle to work well with developers—the positions simply require different modes of problem solving, which can lead to disagreement among coworkers.