I think you mean you see scanlines? interlacing would be drawing alternately odd and even lines every other frame to produce a single image. I'm not aware of a VGA monitor that would have done that. Scanlines are the lines between the row of pixels and you can usually shrink the overall image to make then a bit smaller, but I can't imagine what the dot pitch must be like for someone to even recognize this. I wonder, are you very young and have never used a CRT before? Have you only ever used LCDs?

That monitor is 21-inch, right?
Such big monitors were designed for higher resolutions, so it's perfectly normal you can see scanlines when using standard VGA modes, or even 800x600.

It's normal/expected that modes like 720x400 (VGA text mode) and 640x480 will show scanlines on a large CRT that's been designed for higher resolutions. In order for modes like 1024x768 and 1280x1024 to be legible, the electron beam needs to be quite narrow, making 400-line and 480-line modes have visible gaps between the lines.

Here's an example from my 19" Trinitron. The second photo is showing a 320x200 mode (with 256 colours) where the VGA card is double-scanning, i.e. outputting two scanlines for each pixel. If the VGA didn't double-scan the 200 line modes, there'd be huge black gaps between each scanline even on a modest 14" or 15" display.

Myself, I'm quite fond of the scanline effect. It gives some 'texture' to the output which is missing from modern displays. Note that in these photos, I've cranked the brightness up to 100%, which causes even the black pixels to have a faint scanline visible. Normally I'd run the display with brightness/contrast set so that black pixels are fully black.

Ok, I understand it now. Ty all for explain it.
To Shagittarius: I used CRT (shity 15") in 1997-2005.😀