why there is no incremental backup in sql server
It seems you are coming to SQL Server from Oracle background and hence looking for similar terminology. Incremental backup in Oracle is similar to that of differential backup in SQL Server.
Below is definition of Incremental backup from Oracle:

An incremental backup copies only those blocks in a data file that
change between backups. A level 0 incremental backup, which copies all
blocks in the data file, is used as a starting point for an
incremental backup strategy.
A level 1 incremental backup copies only images of blocks that have
changed since the previous level 0 or level 1 incremental backup.
Level 1 backups can be cumulative, in which case all blocks changed
since the most recent level 0 backup are included, or differential, in
which case only blocks changed since the most recent level 0 or level
1 incremental backup are included.

And below is definition of Differential backup from Microsoft:

A differential backup is based on the most recent, previous full data
backup. A differential backup captures only the data that has changed
since that full backup. The full backup upon which a differential
backup is based is known as the base of the differential. Full
backups, except for copy-only backups, can serve as the base for a
series of differential backups, including database backups, partial
backups, and file backups. The base backup for a file differential
backup can be contained within a full backup, a file backup, or a
partial backup.

Hope above clears your concern.