Why can snap files not be modified in any way?

While the premise of the question is technically correct (you can't change files of a snap), there are ways to work around this.

One such way is to use the --bind option in conjunction with mount, to remount the existing file hierarchy to somewhere else.

For example, if you want your snaps to use the system certificates instead of the certificates installed in core, you can mount the directory containing the system certificates on the host on top of the system certificates directory in core with the following command:

sudo mount --bind -o nodev,ro /etc/ssl/certs /snap/core/current/etc/ssl/certs/

This doesn't actually change the snap filesystem. If you unmount the folder, the old folder will take its place:

sudo umount /snap/core/current/etc/ssl/certs

Note: Mounts do not persist between reboots. There are several ways to make mounts persist after a reboot. One such way is to create a systemd startup script:

$ cat <<-EOF | sudo tee /etc/systemd/system/snap-core-current-etc-ssl-certs.mount
[Unit]
Description=Mount unit to fix etc ssl certs in core package
After=snapd.service

[Mount]
What=/etc/ssl/certs
Where=/snap/core/current/etc/ssl/certs
Type=none
Options=bind,nodev,ro

[Install]
WantedBy=multi-user.target
EOF
$ systemctl enable snap-core-current-etc-ssl-certs.mount

Taken from here.

It's impossible to change the contents of the snap without re-building the snap. This is primarily a security measure, to ensure that the snap hasn't been tampered with.

However, the icon referred to is likely in a desktop file called blender-tpaw_blender.desktop which is editable, and can be found in /var/lib/snapd/desktop/applications.

You could change the following line to update the icon:-

Icon=/snap/blender-tpaw/3/meta/gui/icon.svg