Reason is as per below definition in RFC 4577:
4.2.7.4. Routing and Forwarding on Sham Links
If a PE determines that the next hop interface for a particular route
is a sham link, then the PE SHOULD NOT redistribute that route into
BGP as a VPN-IPv4 route.
Any other route advertised in an LSA that is transmitted over a sham
link MUST also be redistributed (by the PE flooding the LSA over the
sham link) into BGP. This means that if the preferred (OSPF) route
for a given address prefix has the sham link as its next hop
interface, then there will also be a "corresponding BGP route", for
that same address prefix, installed in the VRF. Per Section 4.1.2,
the OSPF route is preferred. However, when forwarding a packet, if
the preferred route for that packet has the sham link as its next hop
interface, then the packet MUST be forwarded according to the
corresponding BGP route. That is, it will be forwarded as if the
corresponding BGP route had been the preferred route. The
"corresponding BGP route" is always a VPN-IPv4 route; the procedure
for forwarding a packet over a VPN-IPv4 route is described in [VPN].
This same rule applies to any packet whose IP destination address is
the remote endpoint address of a sham link. Such packets MUST be
forwarded according to the corresponding BGP route.
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