You can build user defined functions in Excel with VBA as well as with Python through something like xlwings. One of the issues that I ran into with xlwings (or any third party integration into the Office suite) is portability between users.
The ubiquity of Excel is both a blessing and a curse in that everyone has it, so everyone uses it, regardless of whether or not it is the best tool for the job.
Google Colaboratory is now Ubiquitous in the sense you use the term, as is Microsoft Azure Notebooks, so the Ubiquity argument is no longer unique to Excel. The big argument in favor of notebooks is transparency and the breadth of tools that they can make use of. Economists will increasingly move away from Excel as the QuantEcon website demonstrates. Perhaps accountants will still uses spreadsheets, after all they invented them, but it's unclear why anyone else really needs them when there are better tools available.
The ubiquity of Excel is both a blessing and a curse in that everyone has it, so everyone uses it, regardless of whether or not it is the best tool for the job.