For many organizations, the fact that it is free is less important than the fact that they already have Office.

If a team in a mid-to-large company wants to use Airtable, they will have to spend a lot of time championing the process--audits, vetting, bidding, legal, data management, and on and on and on.

If they subscribe to Office365, they've already gone through this process. It will either be enabled by default, or be subject to a much less stringent review before IT flips the switch.

Put another way: money (at least the sums involved for most SaaS) is the least important objection a customer might have to adoption.

Side note: this is also part of the reason why Excel is so popular, even though many executives are aware of its short-commings and might wish for something more integrated. Even if they have the budget, it is excruciating trying to onboard a SaaS offering.