You don't have to.
You can enrol directly on to Tec40 with the following prerequisites:
1. Certified as PADI Advanced Open Water Diver or equivalent.
2. Certified as PADI Enriched Air Diver or equivalent.
3. Certified as PADI Deep Diver... OR... Proof of 10 dives to 30m/100ft.
4. 18 years old.
5. 30 logged dives: 10 with EANx, 7 deeper than 30m/100ft.
Note.... there is an acceptable logged dive experience alternative to the Deep Diver qualification.
This is as per the PADI TecRec 'Tech Deep Instructor Manual', but it isn't publicised in PADI marketing materials...
Beyond this... you NEED TO BE READY to take on the challenge of tech training. Yes, Tec40 gives you a LOT more training than the Deep Diver course... but you need to have the foundational competencies in place if you are to achieve that benefit..
If the Deep Diver course
actually provided those strong foundational competencies them I'd wholeheartedly support it as a prerequisite. But the fact is, when taught to the minimum standards by rec instructors with zero tech diving experience, the course is very weak... and mostly irrelevant as preparation for tech training.
However, do a Deep Diver course with a serious tech instructor... for the purposes of being a primer to tech diving (beginning with the end in mind)... and it could be very beneficial indeed...
As a tech instructor, I'm far more interested in the diving competency of my students. Whilst I have to follow the minimum prerequisites stated by the agency, these are the least of my concerns. All tech instructors are supposed to conduct pre-course assessments... I think a few (or more) are only interested in doing credit checks.. LOL
Here's an article I wrote that compares Deep Diver and Tec40...
PADI Deep Diver versus the Tec40 Course: A Comparison