Why is Thorium (At. no. 90) an f-block element at all?
The bulk metal actually does involve the $f$ orbitals, distinguishing it from what would be its congeners in Group 4 of the $d$ block.  From Wikipedia:

Despite the anomalous electron configuration for gaseous thorium atoms, metallic thorium shows significant 5f involvement. A hypothetical metallic state of thorium that had the [Rn]6d27s2 configuration with the 5f orbitals above the Fermi level should be hexagonal close packed like the group 4 elements titanium, zirconium, and hafnium, and not face-centred cubic as it actually is. The actual crystal structure can only be explained when the 5f states are invoked, proving that thorium, and not protactinium, acts as the first actinide metallurgically.[1]

Cited reference:
1.
Johansson, B.; Abuja, R.; Eriksson, O.; et al. (1995). &quot;Anomalous fcc crystal structure of thorium metal&quot;. Physical Review Letters. 75 (2): 280–283.