Most agencies do choose a niche. They put it on their website and pitch it in conversations. Then someone calls who sits just outside it, with a decent budget, and they say yes.
That isn't weak. It's human. But in one conversation, it undermines what you've spent a long time building. Every exception confirms to the market that you don't have a real position.
In the SHARP method, this is called Hold Your Ground. Choosing a position takes one day. Holding that position is daily work, and it's hardest when things aren't going well. Saying no to a client who doesn't quite fit is the standard in action. Do that consistently and you can build a position in one year that takes a competitor five years.
Most agency owners don't do it because they're afraid the phone will stop ringing. What happens in practice is different: the phone rings less often, but the conversations you do have are better. A shorter sales cycle, higher conversion, clients who know why they're choosing you and don't just want to negotiate on price.
There's another effect you don't see coming: your work gets better. If you keep working in the same sector, you build expertise faster. You see patterns you would otherwise miss. You know what works and what doesn't. That expertise is exactly what separates your position from a generalist who's also giving it a try.
And expertise is what justifies higher rates. Not a better story or a better website. Deeper knowledge of one market, built over years. That's what makes you hard to replace. The Agency Standards give you the framework to document and hold that position consistently.