Because of the way the Mac detects displays, you have to connect the Mac LAST to get the external display to light up.
The Mac does not rely on Windows-like side-loaded "Drivers" which are actually packages of resolutions and settings for a specific display. Instead, it goes straight to the immutable source -- it asks the display itself.
To get a Mac display to become active, you need the Mac to query the display, and the display to answer with its name and capabilities. Otherwise, the display will not be shown as present, and no data will be sent to the display. "No signal detected" is generated by the DISPLAY, not by the Mac.
This query is only sent at certain times:
• at startup
• at wake from sleep — so momentarily sleeping and waking your Mac may work
• at insertion of the Mac-end of the display-cable, provided everything on that cable is ready-to-go
• hold the Option key while you click on the (Detect Display) button that will appear in Displays preferences (from another display)