A coding test needs a witness. An independent inspector has to attend, be booked in advance and be paid whether the candidate passes or fails, so test dates are fixed by somebody outside the school entirely.
Wire, gas and plate are consumed by the hour. Material cost per student is calculable and substantial, and a course priced without it loses money precisely on the students who practise most.
Certificates outlive the course by years. An inspector or an employer may ask for verification long afterwards, so a register of who was tested, on what and by whom is part of the product rather than paperwork.
Expiry is the best marketing list there is. Approvals that lapse on a known date mean every former student is a future booking, and contacting them a month before is more effective than any advertisement.