Because its use has been standardized by my employer.

Exactly, if I send anything other than a .docx to my colleagues, it will be ignored.

LibreOffice supports docx.

Again, if it's your employer's decision, then it's your employer's problem that every price of documentation is leaking outside of the company. Is it safe in a cloud? No one can tell for sure. Maybe your employer doesn't care. For any private usage, LibreOffice works best. It's not trying to sneakily upload your private information to Microsoft.

Will .odt not work?

If it's a company's procedure, then why would you care. They should care, not you. If they are okay with it, it's their problem.

For any private usage, LibreOffice works better. It's not trying to leak everything you do to Microsoft.

As any LibreOffice user will tell you, LibreOffice is hot wet trash compared to earlier versions of MS Office.

LibreOffice suffers badly from a very dated and unfriendly UI. It's an iconic example of large FOSS applications having terrible UI/UX.

Office is quite simply a better product apart from the connection to Microsoft. Or at least it was, I haven't touched it since Windows 10 launched. Either way, LibreOffice is only barely viable as an alternative.