It does increase engineering headcount because you are expected to add a lot of features to your offering that end users wouldn't necessarily care about, like DLP, eDiscovery, audit logs, SCIM provisioning, admin controls, compliance with a hundred industry regulations (HIPAA, FINRA, SOX), key management, custom retention policies, data residency. The larger increase comes from the fact that large companies aren't using the checkout form on your website to buy the product, but expect a dedicated team to negotiate the contract and help with product rollouts and customization across their company.

Because you enterprise customers expect it. They pay large multiples above SMB to get dedicated (human) service