My guess is poor design. Some people like the look of transom windows but do not have the ceiling height to accommodate proportionate space for a transom. They squeeze in a transom window and it looks silly with pain deviders. (mispelling intentional)

Consider cottage style windows instead of transoms.

I went with the title.

Ya, you just lift up the bottom sash and walk into the room.

I lived in a home built circa 1900 which had interior transom windows over bedroom doors to enhance ventilation. Each consisted of a single pane of glass with no muntins. I suspect the majority of interior transoms installed today (including those installed by my company) are a single pane with applied muntins for purely aesthetic purposes.

Our 1890 home in AL had transom windows original to the house including a beautiful stained glass above the front door that had the original brass hardware for opening and closing.

I replicated the idea by buying salvage stained glass as well as just beautiful windows in appropriate sizes and had them installed above several interior doors as well as along the top of an open doorway .

In this 1910 house I had a plain antique window added above the bathroom door for more light into the room as we had to make the window smaller.

Look into salvage windows for real muntins and unique beauty.

I usually prefer the look of a taller window unit instead of a shorter window with a transom above. Ceiling height also needs to be considered and the overall style of the home.

Sometimes transoms are added to the wrong house and it just looks like they were checking off boxes of things to add, and transoms were one of the items on the generic list.

Context is key.........................

Mostly history and aesthetics now, not structure.

Divided panes started because large sheets of glass were expensive and hard to make. Once float glass became standard, dividers became a style choice. Many "divided" units today are a single pane with grilles applied on it or set between the panes.

What actually constrains an interior transom:

Safety glazing. Glass in a door must be tempered or laminated. Glass near a door is only treated as hazardous when its bottom edge is below 60" off the floor (and within the distance-from-door limits). A transom over a standard 6'-8" door usually sits above that line. Confirm against your jurisdiction's adopted code edition.

Framing. On a bearing wall, the header has to span the full opening above the transom. The ceiling height minus header depth caps how tall the transom can be. On a non-bearing wall this mostly goes away.

Sound. Fixed glass blocks sound better than an operable transom, and laminated glass does better still.

A single pane reads cleaner with modern trim. Dividers suit traditional casing and older houses.