I have the same gripe with Actions. I am working around it for now by setting repeated job configurations as inputs that are not required and have defaults. That way they are defaulted but can also be user-overridden if desired. I suppose they could also be workflow-level env vars but that would make them not user-overridable, if that's a concern for you.

So I do:

on:
  workflow_call:
    inputs:
      runs-on:
        required: false
        type: string
        description: The self-hosted runner to use
        default: 'linux'

      timeout-minutes:
        required: false
        type: number
        description: The maximum time in minutes that the job can run.
        default: 90

# note that you *can* set this default
defaults:
  run:
    shell: bash


# use the workflow defaults here
jobs:
  init:
    name: Initialize
    runs-on: ${{ inputs.runs-on }}
    timeout-minutes: ${{ inputs.timeout-minutes }}

The suggestion to use the env context is a good one although its use is limited - For example, I found that the env context can not be used for workflow or job names, only for step names:

on:
  push:
  workflow_dispatch:
    inputs:
      my-var:
        default: abc

env:
  TAG: ${{ inputs.my-var || 'abc' }}

run-name: My Workflow with variable [${{ env.MY_VAR }}]     << does not work

jobs:
  some-job:
    name: My JOB with variable [${{ env.MY_VAR }}]          << does not work
    steps:
      - name: My Step with variable [${{ env.MY_VAR }}]     << works!
    

We can use inputs for the workflow/job names but on push events, those values are empty :-(

An alternative is to use store defaults in Repository/Organization variables that can then be accessed from the vars context - i.e. you could do something like ${{ inputs.my-var || vars.DEFAULT_MY_VAR }}

If there are any other suggestions for using (default) variables regardless of the event type - please post them.

It would be awesome if a push event could inherit the default inputs from the workflow_dispatch event ...

More about contexts and how to use them here