This is as per design - only outbound templates are supported. Refer the official documentation where it states
How to Configure MobileConnect for Use with Journey Builder
steps below to properly configure your MobileConnect messages for use
with Journey Builder:
Contact your Salesforce Marketing Cloud relationship manager to ensure
your account allows access to both MobileConnect (using the upgraded
version of Contacts) and Journey Builder.
Ensure the MobileConnect
Demographic data extension contains the contact data used to
personalize any messages used as part of Journey Builder.
all contacts you wish to participate in your Journey Builder
activities subscribe to the appropriate short or long code and
You can specify those subscriptions at import or by using
messages such as the Mobile Opt-In template.
Create all Outbound MobileConnect messages using the Interaction send method to permit Journey Builder access.
Note that all contacts must initially
subscribe to your short or long code and keyword to receive messages
at the beginning of the journey. The contacts must also remain
subscribed continuously to your short or long code and keyword to
continue receving messages as part of the Journey Builder activity. If
the contact unsubscribes at any time, that contact will not receive
Once you properly configure your MobileConnect messages, those
messsages become available for addition to Journey Builder
Use the Send SMS activity within Journey Builder. Click Configure to
select the message you wish to send within that activity. Journey
Builder will display a list of all messages using the Interaction send