This is an interesting point, because they are kinda infinite spaces but they also impose a structure on the space, which I would assert is a part of why they are so successful, in contrast to the "infinite structureless blank paper" that the OP is talking about.
The trick is getting the amount of structure just right, not too much to be too restrictive and not so little that users are lost in the way the person you're replying to describes.
Infinite canvases require panning and zooming with the mouse and tracking motion visually. A lot of users probably don't like the cognitive load of that compared to other types of apps.
Excel handles this better because navigating around a large sheet feels more like "snapping" as there's no UI motion as your view zooms in and out. Plus with shortcuts like CTRL + rArrow, which immediately snaps your selected cell to the rightmost end of the current range of cells, the infinite canvas feels downright zippy. Excel sheets also have tabs that signal to the users about how they can split up their data instead of filling up Sheet1 with every scenario. Infinite canvases make you create a new file.
You are usually not dropped in the middle of a spreadsheet with infinite rows/cols in each direction. You start at topleft which makes navigating much less 'exploratory'
If I recall correctly, Lego Mindstorm's GUI-based programming application (along with its various iterations) did something similar too, starting at a "leftmost" side of an infinite canvas.
