The article linked below may not give the clearest anwers to your question of why Dr. Perez left LAC, apparently back in 2013 as best I can ascertain, but it does have some interesting language that I have seen so many other companies use while they are in a development status.  It seems there is a standardized set of expressions that these companies use in order to asuage the savage beast:  the wrath of stockholders.

“On behalf of the Board, I would like to thank Waldo for his major contributions, and wish him success in his new ventures."   A pretty standard exit statement from a company upon the departing move of one of the officers of that company.

The paragraph below seems to have been almost written by a speech writer paid by any small cap company "in development" in order to apease the beast:

"Also Wednesday, the company updated investors on project financing, after creating an executive committee to oversee various financing proposals and to manage discussions with potential strategic partners last November. Since then, the company said it has had "numerous" parties review contents of its data room, with the process generating "strong and continuing interest from a number of potential partners" globally, despite the difficult financing climate."

Notice the use of:  "potential strategic partners" and "strong and continuing interest".

“We are committed to running a thorough and professional process in order to execute a financing arrangement which will maximize shareholder value,” said Hodgson in the statement.  “We are hopeful to be in a position to provide more specific detail on our project financing discussions in the near future.”

This was back in 2013, 8 years ago... and what about today?  LAC is still looking for a "financing arrangement which will maximize shareholder value".  How many times with other small comapnies in development have I seen the CEO of the company sprinkle that same Pixie dust phrase:  "maximize shareholder value"?  I think it is part of the manual that comes as a mandatory training point for any CEO of a Small Cap company "in development".  

Well, it has taken 8 years, but Hodgson has maximized shareholder value, at least from what it was a mere 6 months or less ago.  We still, however, are looking for that "strategic partner", at least as it applies to LAC's Thacker Pass development.

I think that the take home lesson from looking back 8 years to these statements is as it applies to all small cap companies "in development":   The CEO will always pull on the heartstrings, more like the ROI hopes, of the stockholders by using that "maximize shareholder value" phrase and that they will always be looking for a sugar daddy in the form of a "Strategic Partner".  Seems to me, just my opinion, that these are necessary word tools for any small cap company in develpment... along with others such as:  "Shelf Offering", stock dilution and stock option granting to all the company officers.   Just part of the game!

We are fortunate that LAC has come through the minefield provided for all small cap companies in development and stands tall in its present position, even though we are still looking for that "Strategic Partner".  It sounds like I am complaining, but I am acutally just noting my obervations about the process involved in getting to this point and I remain quite positive about the rest of the journey with LAC, but as I look at my other investments, also with small cap companies "in development" for the most part, I see similar statements come out from their own CEOs as if they all are reading from the same playbook.   

Lithium Americas rejigs senior management team as current CEO to step down | TSE:LAC (proactiveinvestors.com)

The link below indicates Dr. Perez left LAC in 2013 "to pursue other business interests", again a pretty standard phrase when an upper management member leaves.

Lithium Americas rejigs senior management team as current CEO to step down | Proactive Investors NA (wordpress.com)

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Shedding more light on possibly why Waldo Perez left LAC for Neo Lithium can be found in a slickly produced obviously highly biased article from Small Cap Power by Keith Schaefer.  

Keith Schaefer, Investing Whisperer | November 24, 2020

Neo Lithium Looks Like a Top Lithium Junior Speculation (smallcappower.com)

Schaefer does disclose that he is an investor in Neo Lithium in the article, so his bias is clear.  The story he gives about Dr. Perez finding the only lake in South America with a high lithium brine content is designed to suck in otherwise uninterested readers as to the exploration skills of Dr. Perez.  It also has a "Where's Waldo" component to it in that Dr. Perez found the lake by accident while wondering around in the Andes.  So, it would appear that Waldo's trek through the Andes constituted his pursuit of "other business interests".  Bottom line seems to be that we should be thankful for the role Dr. Perez in the early days of LAC even though he has gone on to other things.  No doubt he will have much success with Neo Lithium, but so will Jon Evans have with LAC.

Okiedo