Yes we can use a calculated field for last activity to rollup the max date of a contact or account or lead easily (see pic of how in my post below). But the rollup field doesn't allow you to filter the owning business unit. It is very common to have more than one unit in the crm so we need easy way to rollup max activity at account, lead and contact level of activity and related rollup activities.

So this rollup example for an account works except when you try to filter owning business unit you only get contains data or not.

You can see example pic of the rollup field setup in my post, but stuck on filtering owning business unit: Update Last Activity Dynamics 365 of owning busine... - Power Platform Community (microsoft.com)