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.

In addition to Windows simply not having support for Linux file systems, Microsoft cannot reasonably take on the responsibility of guaranteeing your data will be safe if they try to support them. For example (and completely hypothetical), say the ext4 team improves performance by recognizing a new ext4 inode format that is not backward compatible. Windows next modifies your data but mixes the old format with the new one, because it doesn't know about the change, and corrupts your data. Who will you blame?

For years, as my memory recalls, the Linux kernel had read-only support for NTFS, and write support was labeled UNSUPPORTED for years after it was introduced. The Linux kernel was obviously very concerned about data integrity until they were confident their NTFS file system support was fully stable and understood. Now imagine how much harder it would be on the kernel team if Windows had 3-5 other filesystems just as popular as NTFS. (Think: ext3-4, XFS, ReiserFS, Btrfs, etc.)

Its just because of the difference in File systems of Windows and Linux systems. Windows does not support the Linux files or file systems.