By default, System Activity data is stored in Looker's internal database. Most tables are truncated on a regular schedule to comply with storage limits. For example, the History table is truncated to the past 90 days of data.
Field Usage Article (<your-hostname>/explore/system__activity/field_usage) is for Lookml fields and the number of times they have been queried from any source. Underlying field_usage table is a type of table which records queried fields from all time and doesn't have any retention policy which means the data will not be removed after a certain time line due to it not being an events based table.