Why does Windows not recognize files inside Linux partitions?

Windows does not have native Linux filesystem support (ext3, ext4, zfs, among others). It's as simple as that.

Windows only natively supports the NTFS and FAT (several flavors) file systems (for hard drives/magnetic systems) and CDFS and UDF for optical media, per this article.

To access other file systems, additional drivers/software will be required. As an example, the Ext2 Installable File System For Windows driver is an open source project that supports the Ext2 system.

EDIT: Correct typo - listed "UFS" instead of "UDF" - kudos to @ChrisS for catching the mistake.