Wi-Fi signals spread like light from a lamp. The further you are, the dimmer they become, and every wall, floor and piece of furniture absorbs some of the signal. A router tucked in a corner or behind a television is already starting at a disadvantage.
Vertical distance is especially hard. A router on the ground floor sends most of its signal outward, not upward, and ceilings and floors block more than interior walls. Upstairs rooms often sit in a radio shadow even when they are not far away.
The 5 GHz band is much more affected by obstacles than the 2.4 GHz band. A device that stubbornly holds onto a weak 5 GHz signal will be slower than one that drops to 2.4 GHz, which is why band steering can sometimes make things worse.
The fix is either to move the signal source closer or add another source. A single router cannot push a strong signal through every wall of a large house, which is why mesh systems and extenders exist.