They can, and many do. The problem that emerges at scale is the same one that appeared with direct LLM provider connections: no visibility, no access controls, and no shared place to attribute usage or enforce policy. An agent calling an MCP server directly handles its own credentials, defines its own tool scope, and produces no shared log. When you have dozens of agents across multiple teams all calling tools directly, you have no way to see what's being called, by whom, or at what cost — and no mechanism to prevent an agent from calling a tool it shouldn't. MCP Gateway is the control point that makes tool access observable and governable without changing how agents are built.