In short: because of SharePoint.
Several years ago the Dropbox desktop client has been running on my computer, and I could interchange documents for free, without accessing the browser interface of this popular cloud-based service. Then I've uninstalled the desktop client, and I've continued using Dropbox as a personal file sharing service, because my clients preferred Google Drive or OneDrive for teamwork.
Then I've learned: "There's no such thing as a free lunch". Technically it's easier to extend the existing data centers with server and data storage resources, than increasing the throughput of the networks for accessing those servers - consequently the companies offering cloud-based services are still offering for each subscriber a limited amount of free cloud storage, but the efficiency of using that free storage is highly variable.
The stick has always to ends, so if my file uploading or downloading experience is poor, and my cloud-based file sharing service becomes unresponsive or is throwing weird error messages, the problem could be at their end or at my end.
For example my local ISP has to deal with high traffic during our evening hours, and I've learned that uploading big files (more MBs each) does take many more time during peak hours. After getting confused when Dropbox has failed to upload my file (less than 1 MB) during peak hours, I've learned that I need to double-check if my files have been uploaded successfully into Dropbox, and if not, I have to repeat the operation after 30-60 minutes or so.
After getting the feeling of how things work with an Office 365 Enterprise subscription, I've understood the difference between free and paid file sharing, and I understand those people, who just don't want to look back.
Beyond the fact that one's files are always getting when and where they need to be, more persons can losslessly update different sections of the same file in the same time, and a refined system of access rights becomes available: for example deleting, forwarding or downloading a file can be permitted or denied.
Small companies and/or teams have various cloud-based file sharing alternatives to choose from, and testing more of them is the best way to find out, which would work better for their needs.
The free OneDrive service has the advantage, that after upgrading to a commercial package, SharePoint can be plugged in and configured to suit higher security and heavier file sharing needs.