Because they install crappy motherboards, crappy power supply units, crappy memory modules, it's always like "hey dude look at the CPU and GPU, look it's 16 GB RAM, and better don't look at the other parts, give me your money instead!".
Choose all the right parts, find a video on how to connect everything in it, for example
and then do it by yourself.
Just an example of a custom-built PC:
CPU: Ryzen 3600 BOX
RAM: Crucial 2*8 GB Ballistix Sport LT 3200 MHz or 3000 MHz if no money
HDD: I would start from a regular 1 TB 7200 rpm HDD, e.g. WD10EZEX. Yeah I know all those cocksuckers say it's not cool to have no SSD in 2019 blablabla, it's up to you, having no SSD does not prevent me from having 300 fps in the FPS_BENCHMARK with the highest graphics settings, the only noticeable frametime spikes I get are in warmup of competitive MM.
PSU: Corsair RM650x (CP-9020178)
MOBO: ASUS PRIME X470-PRO or ASUS TUF B450-PRO GAMING (ask the store you're buying it from or an ASUS service center in your city to update your BIOS since only X570-based motherboards support Ryzen 3000 out-of-the-box)
GPU: I'd start from at least GeForce GTX 1660.
You bet I haven't checked the prices in your country, do it by yourself.
It's just an example of a PC you might buy, I'm sure it costs more than 700e, and if you can't get more money from your parents or just by saving more from your salary - buy whatever you can, personally I saved money for 3 months to buy a mid-low-end PC for ~1200e.